Opinion for the Court filed by Circuit Judge MacKINNON.

Upon complaint of the Anaconda Company, Review Board Number. Four of the Interstate Commerce Commission (ICC) reduced the rail rate for shipping aluminum ingots from Conkelley, Montana to Terre Haute, Indiana (“the Terre Haute route”). In prescribing a lower rate to Terre Haute the Board relied upon the rate that the defendant railroads were charging another aluminum company for shipping the same product over a longer, overlapping route. Because the Board failed to show that the rate for the longer route provided a proper measure of the reasonableness of the Terre Haute rate, and failed to consider the full ramifications of a rate reduction, we vacate its decision and remand for further proceedings. We affirm other aspects of the Board’s decision and find it unnecessary at this time to address the railroads’ contention that the decision must be vacated in any event in light of the intervening enactment of the Staggers Act. We leave the initial application of that Act to the Commission.

I. BACKGROUND

Anaconda produces aluminum ingots from alumina at its smelter at Conkelley, Montana. Most of Anaconda’s alumina is shipped by water from the Caribbean island of Jamaica to the port of Everett, Washington, and then shipped by rail to Conkelley. The aluminum ingots produced at Conkelley are transported by rail to Ana*54conda’s aluminum rolling mill in Terre Haute, Indiana, where they are fabricated into mill products such as aluminum sheet and foil.

The ingots are transported by the Burlington Northern Railroad from Conkelley to Chicago, and by the Louisville & Nashville Railroad from Chicago to Terre Haute. These carriers also transport aluminum ingots of the Martin Marietta Aluminum Company from Cliffs, Washington to Lewisport, Kentucky (the “Lewisport route”). The Lewisport' route runs over much of the same track as the Terre Haute route, but is longer at both ends-(a total of 2448 miles as compared with 1757 miles).

In September 1978, Anaconda filed a shipper’s complaint with the ICC, seeking a 38.8 percent reduction in the rate of $52.76 per ton charged by the railroads for shipping aluminum ingots to Terre Haute. One of Anaconda’s allegations was that the rate was unreasonably high, in violation of the Revised Interstate Commerce Act.1 Before the Commission can reduce a rate for being unreasonably high there must be a showing that the railroad has “market dominance” over the traffic in question. 49 U.S.C. § 10709.2 The Administrative Law Judge (ALJ) noted that “defendants do not contest the allegations [concerning] their market dominance, and, in fact, confirm it by their submission of cost evidence which reveals that their revenues from complainant’s traffic exceed 200 percent of their variable .costs.” 3

Although finding for Anaconda on the threshold issue of market dominance, the ALJ held the railroads’ $52.76 rate was not unreasonably high. Anaconda had urged the ALJ to rely on a comparison between the $52.76 rate for shipping aluminum ingot over the 1757 miles of the Terre Haute route with the $48.60 rate the same railroads charged for shipping the same product over the 2448 miles of the overlapping Lewisport route. ■ The ALJ declined to rule that the Terre Haute rate was unreasonably high, however, apparently because the seeming anomalous disparity between the rates had resulted from a series of general rate increases authorized by the Commission but not fully implemented by railroads in each of the rate territories concerned.4 The railroads in the territories covering the Terre Haute route had consistently taken advantage of all the authorized increases, but the railroads in the Southern Territory, in which Lewisport is located, had occasionally waived their entitlement to the rate increase.5 The ALJ did rule for Anaconda, however, on an independent ground: the disparate rate structure created an undue preference in behalf of the Martin Marietta Aluminum Company of Cliffs, Washington.6 *55Both sides appealed the decision to the Commission.

In October 1979, the Commission’s Review Board .Number Four overruled two conclusions of the ALJ.7 In one instance the Board ruled in favor of the railroads, dismissing Anaconda’s claim of undue preference.8 The other ruling, in favor of Anaconda, is the subject of this appeal. The Review Board affirmed without discussion the ALJ’s finding of market dominance, but rejected his conclusion that the $52.76 rate, Conkelley to Terre Haute, was not unreasonably high.

The basis for the Board’s holding was set forth in some detail. The Board explained that in determining whether a rate for shipping a given commodity over a given route is unreasonably high one should consider (1) the ratio of the revenue generated by the rate to the variable cost of moving the relevant commodity over the relevant route,9 (2) the value of the commodity, (3) rate comparisons, (4) rail revenue need, and (5) the effect of a rate reduction on the transcontinental freight rate structure.

The Board observed first that the railroads’

revenue-to-variable cost ratio of 250 percent (218 percent at the revenue need level) is a high ratio. However, the Commission has long held that costs alone do not determine the maximum limits in ratemaking and that rates which exceed even fully allocated costs by substantial margins do not necessarily exceed maximum reasonable rate levels solely by reason of that fact. Fibreboard or Pulpboard, Montana to California, 357 I.C.C. 211, 217 (1977). Furthermore, rates resulting in revenue-to-variable cost ratios greater than 200 percent have been approved in several cases. See, Agrico Chemical Co. v. Seaboard Coast Line R. Co., 361 I.C.C. 333, (1979) and the cases cited therein. Therefore, we must look at other factors to determine if the rate is unreasonable.10

*56Three of the other four factors the Board considered relevant were either neutral or supported maintenance of the railroads’ rate level. First, in relation to the value of the commodity (aluminum ingots), the Terre Haute rate from 1973 to 1977 had decreased from 6.1% of the selling price to 4.7%. The Board called the value of the commodity “important because differential pricing requires some commodities to carry higher rates than others.” 11 The increase in the commodity’s value in terms of the transportation cost “indicate[d] that aluminum may be better able to bear a high burden than would many other commodities and still move. However, the rate comparison, discussed below[,] indicate[s] that in comparison to other movements of ingot[s], the burden is disproportionately high.” 12

The other two factors — revenue need of the railroad and the possibility of rate disruption — also would have tended to support a maintenance of the rate level had the evidence borne them out, but the Board held it did not: “The present rate levels exceed fully allocated costs by a very high percentage, even at a revenue cost level”; and a reduction in the Terre Haute rate would not “require a complete adjustment of all Pacific Northwest rates with a corresponding decrease in overall revenues.”13

The last of the remaining factors — rate comparisons — indicated to the Board that the Terre Haute rate was unreasonably high. The Board rejected the railroads’ proposal to compare the Terre Haute rate with rates on other movements of aluminum ingots from the Pacific Northwest to Indiana, reasoning that “the routes of movements and carriers used are not necessarily the same, therefore the cost may also differ based on these factors and the greater distance.”14 The Board did find reliable, however, a comparison between the Terre Haute route and the Lewisport route inasmuch as they had a common track and shared other similar transportation characteristics. The Lewisport route was forty percent longer than the Terre Haute route, yet its rate of $48.60 per ton was about eight percent lower. The Board further noted that “[p]rior to the varying [general] percentage increases, the rates from Conkelley [to Terre Haute] were approximately $3.59 per net ton (13 percent) lower than those from Cliffs [to Lewisport].” 15

*57The Board then decided that, “[wjeighing these various factors, ... a rate not to exceed $42.41 per net ton . . . represents the maximum level of reasonableness.”16 The Commission prescribed this rate on the grounds that it would yield a reasonable rate of return and that it would “also correspond with the rate relationship which existed between the Conkelley-Terre Haute and Cliffs-Lewisport movements before the application of varying percentage increases when the assailed rates were 13 percent lower.”17

While reviewing the ALJ’s opinion on the separate issue of whether the Terre Haute rate was unduly preferential, the Board reviewed the railroads’ criticism of the rate comparison. The railroads had argued that differing territorial applications of the general rate increases were based on “different circumstances and conditions of the various carriers, including their revenue needs and costs of operation, as well as the competitive forces they faced.”18 The Board reasoned that varying percentage increases are not differences in transportation conditions justifying a rate disparity inasmuch as those variances reflect general transportation conditions and “[i]t is the transportation characteristics of the particular movements which are relevant.” 19

Subsequent to the Board’s decision, Congress passed and the President signed the Staggers Act, Pub.L.No. 96-448, 94 Stat. 1895 (1980), which narrows the scope, of federal regulation of the railroad industry. Prior to the Staggers Act, the Commission permitted a shipper alleging unreasonably high rates to meet its threshold burden of proving the carrier’s market dominance by relying on a rebuttable presumption of market dominance, which was triggered by a showing of a 160 percent revenue-variable cost ratio, a 70 percent market share, or a substantial investment. Among the changes wrought by the Staggers Act was a removal of Commission rate jurisdiction over rates that have a revenue cost ratio of under 160 percent.20

The railroads in their petition for review argue that the Board’s decision lacks reasoned findings, violates its mandate under “the new rule of ratemaking” to consider the need for adequate rail revenue, fails to consider the adverse effect that rate reduction in this case will have on railroad eam*58ings, and must be reversed in any event in light of the intervening passage of the Staggers Act.

II. DISCUSSION

We must sustain the Review Board’s finding of an unreasonably high rate if it is rationally based upon substantial evidence, giving due regard to its skill and experience in determining the reasonableness of rates.21 We set aside the order and remand for the reason that the Board appears to have given inadequate consideration to the full implications of two of the factors on which it appears to have placed material reliance — (1) the comparison between the Terre Haute and Lewisport rates, and (2) the possibility of a general rate disruption seriously affecting railroad revenue.

A. The Rate Comparison

Only two of the factors considered by the ICC — the revenue-cost ratio and the Terre Haute-Lewisport rate comparison — indicated a rate that was unreasonably high, and the Board acknowledged that “rates which exceed even fully allocated costs by substantial margins do not necessarily exceed maximum reasonable rate levels solely by reason of that fact.” 22 The Board further revealed its material reliance on the rate comparison by prescribing a new rate based on precisely the same disparity (in terms of percentage) that had existed between the Terre Haute and Lewisport rates prior to the series of general' rate increases.23 The validity of the Board’s finding and remedy thus stands or falls with the validity of the rate comparison.

The Commission may rely upon comparisons with other rates in determining whether the assailed rate is reasonable.24 For any rate comparison to have probative value, however, the rates must be comparable. The Commission and reviewing courts have consistently insisted that the complainant, who bears the “onus of establishing the unreasonableness of particular rates,”25 must lay the predicate for a rate comparison by showing that the rates being compared to the assailed rate apply to routes that involve similar terrain and other transportation conditions affecting cost.26 Once that predicate is laid, the decided ICC cases suggest (although not unambiguously) that the burden of producing evidence shifts to the carrier, who may seek to show that the compared rate is inappropriate as a benchmark because, for example, it is the result of different competitive conditions.27 Once *59its probative value is properly assessed, the rate comparison becomes one of the several factors the Commission may consider in making its finding and prescribing, if necessary, a new rate.

The Review Board here used the comparison between the Terre Haute and the Lewisport rates both as a factor to indicate that the Terre Haute rate was unreasonably high and as the basis for its rule-of-thumb prescription of a new rate. In both instances, we conclude, the Board acted arbitrarily and capriciously.

There is no doubt but that the claimant, Anaconda, established that the transportation conditions (the factors affecting costs), for the two rates were substantially the same, involving as they did the same commodity and much the same track. This showing established a rebuttable presumption that the rate comparison was a valid indicator of the rate’s reasonableness. Indeed, several ICC eases state that a rate is prima facie unreasonable when it exceeds another rate applying to carriage of the same commodity in the same direction over the same track for a greater distance.28

The question, then, is whether the Board adequately considered the record evidence that the rates were nevertheless not comparable. The Board’s expert judgment is entitled to great weight, of course, but we must first be satisfied that its judgment has been applied. We conclude the Board did not give appropriate consideration to the railroads’ claim that “competitive forces” made the two rates not truly comparable.

Anaconda offered no evidence on this point. The evidence pointed to by the railroads includes the facts (1) that the rate disparity developed as the result of the decision of the Southern Territory railroads to waive totally or partially the general rate increases awarded to the Western and Eastern railroads 29 (2) that during 1972-77 the movement of primary aluminum pig, slabs, or ingots from the Mountain Pacific to the Official Territory (in which Terre Haute is located) increased by 14 percent while the tonnage moving from that origin to the Southern Territory decreased 32%, despite the relative reduction in the Southern Territory rates,30 and (3) that Pacific North*60west aluminum companies receive their raw materials by water-rail or water-motor carrier transport and might ship to Southern Territory plants by way of the Panama Canal and the Mississippi and Ohio rivers.31

This evidence suggests, albeit somewhat tenuously, that the Lewisport rate was depressed for competitive reasons, a circumstance that would undermine the validity of the Terre Haute-Lewisport rate comparison.32 Although the evidence is obviously circumstantial rather than direct, we do not think it can be ignored or rejected as irrelevant. The evidence may possibly be interpreted consistently with the view that the competitive circumstances surrounding the Terre Haute and Lewisport rates are similar. The Board, however, has not addressed this evidence and advanced that interpretation, and we cannot indulge in the assumption that it would do so without usurping its function. The railroads’ competition claim, although not necessarily compelling, left Anaconda, as the party bearing the ultimate burden of persuasion, with the task of rebutting that claim.33 And the Commission, once the issue of competition is raised, should explain why it finds the rates nonetheless comparable.

The railroads’ further complaint with the Commission’s reliance on the rate comparison is that, assuming some reliance was justified, the Board was arbitrary and capricious in prescribing a maximum reasonable level on the basis of the 13% disparity that had prevailed between the two rates prior to the varying territorial applications of the general rate increases. The railroads contend that where, as here, the Commission is prescribing a new rate on the basis that it represents the maximum reasonable rate, and where, as here, the level prescribed is based on the level of another rate, the prescription is invalid absent a finding the compared rate is itself at the high end of the zone of reasonableness.34 The railroads cite for that proposition Cargill, Inc. v. Chicago, M., St. P & P. R. Co.: “A comparison of the assailed rate with one other rate is no indication that the former is unreasonable, for it may well be that the latter rate is below a maximum reasonable level instead of the assailed rate being unreasonably high.”35 They also cite the statement of the Supreme Court in Youngstown Sheet & Tube Co. v. United States that rate comparisons are “not a conclusive *61test of reasonableness of a rate under investigation.” 36

The Board did not explain how its reliance upon the Lewisport rate in setting the maximum reasonable level of the Terre Haute rate was reconcilable with the logic of these cases, and we do not. find them sufficiently distinguishable to allay our concern that the Commission may have arbitrarily departed from precedent.37 Justifications for the Boards’ determinative reliance on the Lewisport rate may exist,38 but they do not appear in its decision.

The Board on remand should reexamine its use of the rate comparison, both as an indicator of unreasonableness and as a measure of the maximum reasonable Terre Haute rate.

B. Revenue Adequacy Under the Staggers’ Act

The railroads also assert the Board’s decision contravened the dictates of the “new rule of rulemaking,” specifically 49 C.F.R. § 1109.25, which states, “the need for revenue adequacy will be taken into account and regarded as a highly important factor.” 39 According to the railroads, the Board ignored the railroads’ need for differential pricing. They contend that, in light of the decrease in the Terre Haute rate as a percéntage of the selling price of aluminum ingots, this “is the paradigm situation for approval of a rate well in excess of costs to make a contribution to the overall revenue adequacy of the railroads.” 40

We do not accept the railroads’ contention. The legislation which inaugurated the new rule of ratemaking did not strip the Commission of its power and duty to protect shippers from the unreasonably high rates of carriers with market dominance. Thus, while the need for differential pricing is something the Commission cannot ignore, it is not the only factor the Commission is *62entitled to evaluate.41' It may continue to consider the factors it has traditionally examined in determining whether the rate of a carrier with market dominance is unreasonably high. In this case the Commission explicitly evaluated the value of the commodity and the need for differential pricing. See note 11 supra and accompanying text. It prescribed a rate that would return 201% of variable cost at the embedded debt level, and 138% of fully allocated cost at the revenue need level. This latter figure reflects differential pricing. Given the Commission’s explicit consideration of the relevant factors and a result that is not clearly unreasonable we cannot say that the Board’s action was arbitrary and capricious under the standards applicable to this phase of its considerations.

C. Possible Disruption of Rate Structure

The railroads’ final objection to the decision of the Board is that it failed to consider the disruptive effect that a reduction in the Terre Haute rate might have on the railroads’ overall earnings.

In declining to find assailed rates unreasonably high the Commission has occasionally relied upon its prediction that prescribing a lower rate would disrupt rate structures and undermine the railroads’ ability to earn revenue and thus provide services to the shipping public.42 An inappropriate reduction would have this effect, the Commission has stated, either because the reduction would provoke destructive price competition or because it would lead to successful demands for rate reduction from other complainants in the same transportation group.43

In their appeal to the Review Board the railroads asserted that

Anaconda’s rates fit into the established Transcontinental rate structure on aluminum ingots, and granting the relief it seeks here threatens a well established and successful rate structure. The rate reduction Anaconda seeks cannot be confined to Anaconda alone, and as defendants’ witness Cobb explains, the cascading effect from granting Anaconda the relief it here seeks would cost the railroads millions of dollars annually.44

The Board addressed this concern:

Nor will an adjustment in the Conkelley-Terre Haute rate require a complete adjustment of all Pacific Northwest rates with a corresponding decrease in overall revenues. The rate structure has already been disrupted by the lower combination rates which can be constructed over the Southern Freight Association points from Washington origins. Greater distances *63are also involved from those origin points which would justify a higher rate. Therefore, we do not find defendants’ revenue need arguments persuasive.45

Read in context, the Board’s statement refers to routes originating in Washington and terminating in Indiana and points east. Those were the routes the railroads had unsuccessfully urged as comparable routes. The Board’s explanation that the rates for those routes would not be subject to any domino effect emanating from a Terre Haute rate reduction was rational and the railroads do not contend otherwise.

The railroads do contend, however, that the Board’s consideration of the effects of rate reduction was inadequate insofar as it “totally ignores the disruption which will be caused in the transcontinental freight structure from Pacific Northwest origins to the destinations west of Terre Haute in Western territory.”46 Counsel for the Commission submit that “all” in the Board’s phrase “complete adjustment of all Pacific Northwest rates” must include rates restricted to the Western territory.47 As we have noted, however, the most natural reading of the Board’s statement is that only the destinations in the vicinity of Terre Haute were considered.

Counsel for Anaconda submit that its plant at Conkelley is the only aluminum smelter in Montana, a fact that makes the Terre Haute rate sui generis. The rates that the railroads brought to the Board’s attention qll involve aluminum plants situated 500 or more miles to the west of Conkelley. Anaconda suggests that the greater distance and more difficult terrain applicable to routes originating from Pacific Coast states would make the Terre Haute rate a poor candidate for a rate comparison in any proceeding challenging the rates for these routes. Indeed, Anaconda continues, the inherent difference between routes originating in Pacific Coast states and those originating in Montana explains why their respective rates are placed in two different tariffs. Finally, Anaconda contends that the evidence submitted by the railroads on routes ending in the Western Territory was inadequate because only the rate for a minimum of 60,000 pounds per carload was shown whereas the Terre Haute rate in question applies on a minimum weight of 180,000 pounds.

However persuasive Anaconda’s explanations may be, they are post hoc explanations of counsel and cannot substitute for the agency’s own explanation. The Board did not address the possibility of rate disruption as to routes ending west of Terre Haute, although the railroads produced some evidence concerning it. The Board will have an opportunity to supply its explanation as to those rates on remand.,

In supplemental briefs filed following the enactment of the Staggers Act,48 the parties have discussed whether it should apply retroactively to this case, which was pending appeal when the Act became law. The railroads argue, among other things, that the Act deprived the Commission of jurisdiction because (1) the ALJ and Board in determining the “market dominance” of the railroads relied exclusively upon the showing that the rate’s revenue-variable cost ratio exceeded 160 percent, a showing that triggered a rebuttable presumption of market dominance under 49 C.F.R. § 1109.1(g) (1979) and (2) that regulation was “clearly at odds with the Staggers Act” and has now been repealed. Counsel for the Commission and Anaconda respond that the Act should be applied prospectively only, and that in any event it would not affect the result here.

As we have done on other occasions,49 we. decline to address the applicability vel non of the newly-enacted statute, and leave its *64initial application to the Commission on remand.

Judgment accordingly.