opinion: Mr. Justice Douglas
  delivered the opinion of the Court.
  Sulphur mined near Galveston, Texas, can be shipped to Danville, Illinois, either by rail or by barge and rail. If the sulphur goes by barge and rail, it is transported up the Mississippi via New Orleans to East St. Louis and then by rail to Danville. The total charge for that movement is $9.77 per ton. The total of the various local rates for all-rail shipments from the mines to Danville is $11.68. But the railroads have established a joint all-rail rate of $9,184 which is lower than both the combination all-rail rate and the combination rail-barge rate.
  Appellants, who are water carriers, requested the competing railroads to establish a joint rail-barge rate of $7.67 on sulphur from Galveston to Danville. The railroads refused. Appellants thereupon filed a complaint with the Interstate Commerce Commission, alleging that the existing rail-barge rates on sulphur were excessive and unreasonable, that through rail-barge routes and joint rates with reasonable differentials below the all-rail rates should be established, and that the refusal of the railroads to establish such joint rates discriminated against the barges as connecting carriers in violation of the Interstate Commerce Act, as amended by the Transportation Act of 1940. Appellants requested the Commission to establish a through rail-barge route and a joint rate and suggested that the joint rate be fixed at $7.67. Appellants proposed that the Danville railroads receive $2.26 as a division of that rate, calculated to be the same as they receive from the all-rail rate from Galveston to Danville. Under the proposed rate, the cost of rail-barge shipments from the mines to Danville would be $9.17 as compared with the all-rail rate of $9,184.
  A Division of the Commission dismissed the complaint, one Commissioner dissenting. 287 I. C. C. 403. The Commission affirmed the Division, three Commissioners dissenting. 291 I. C. C. 422. A three-judge District Court sustained the Commission. 129 F. Supp. 28. The case is here on appeal, 28 U. S. C. §§ 1253, 2101 (b), 2325.
  Section 3 (4) of the Act provides that “All carriers subject to the provisions of this part . . . shall not discriminate in their rates, fares, and charges between connecting lines . . . Section 3 (4) defines “connecting line” as including “any common carrier by water subject to Part III.” Appellants are common carriers by water within that definition. They maintain that it is unlawful under § 3 (4) of the Act for a railroad to refuse to join in through routes and joint rates with a water carrier when it has already joined in such routes and rates with a connecting rail line. They further maintain that the power of the Commission under § 307 (d) to establish through routes and joint rates should have been exercised here.
  We had a closely related question before us in Interstate Commerce Commission v. Mechling, 330 U. S. 567. In that case we invalidated an order of the Commission which approved higher rail rates for the transportation of grain east of Chicago if it had arrived in Chicago by barge, rather than by rail. We reviewed the history of the Transportation Act of 1940 and concluded that that Act “unequivocally required the Commission to fix rates which would preserve for shippers the inherent advantages of barge transportation: lower cost of equipment, operation, and therefore service.” Id., at 575. We held that the discrimination which was outlawed applied to through rates as well as to ordinary rates.
  The Mechling case involved an attempt to deprive water transportation of one of its “inherent advantages,” as that phrase is used in the preamble of the 1940 Act, by increasing the cost of barge service. The Commission’s present decision achieves the same result through the device of a joint rate allowed carriers by rail but denied carriers by water. It was recognized in the debates on the bill that became the Transportation Act of 1940 that manipulation of rail rates downward might deprive water carriers of their “inherent advantages” and therefore violate the Act. It was emphasized that one of the evils to be remedied was cutthroat competition, whereby strong rail carriers would reduce their rates, putting water carriers out of business. There was recognition that for shippers to get the benefit of the “inherent advantages” of water transportation there frequently would have to be joint rail-barge rates. Barge transportation frequently covers only one segment of the journey to market. The failure of the railroads to establish nondiscriminatory joint rates with barges might, therefore, seriously impair the development of barge service as a vital component of our national transportation system. Section 3 outlaws discrimination in all its forms. See New York v. United States, 331 U. S. 284, 296. Where there is discrimination by the use of a joint rate to favor rail carriers over carriers by water and to deprive the latter of their “inherent advantages,” the Commission has a duty to end it under §307 (d). That subsection makes it mandatory for the Commission to establish through routes and joint rates “whenever deemed by it to be necessary or desirable in the public interest.” The public interest, as defined in the Act, is the guide to the Commission’s action. McLean Trucking Co. v. United States, 321 U. S. 67, 82. The policy is to preserve all the “inherent advantages” of the water carriers. That means that a joint barge-rail rate must be established when it appears, as here, that a joint rail rate discriminates against the water carriers. Otherwise a manipulated rate structure will take the business from the water carriers. In absence of the joint all-rail rate, the rail-barge combination rate for sulphur would be $1.91 per ton less than the all-rail combination rate between the same points. That differential reflects the lower cost of the barge segment of the journey. It is at once lost to shippers when joint rates are allowed rail carriers and withheld from the water carriers. To hold otherwise would be to sanction a rate structure which, through the use of discriminatory joint rates, denies shippers the “inherent advantages” of water transportation.
  Reversed.
  The charge consists of rail and loading charges of $1.50 at Galveston, the barge rate of $5.32 to East St. Louis, and the local rail rate of $2.95 from East St. Louis to Danville.
  For a discussion of joint rates see St. Louis S. W. R. Co. v. United States, 245 U. S. 136, 139, note 2; United States v. Great Northern R. Co., 343 U. S. 562.
  24 Stat. 379, as amended, 54 Stat. 898, 49 U. S. C. § 1.
  Section 307 (d) provides:
  “The Commission may, and it shall whenever deemed by it to be necessary or desirable in the public interest, after full hearing upon complaint or upon its own initiative without a complaint, establish through routes, joint classifications, and joint rates, fares, or charges, applicable to the transportation of passengers or property by common carriers by water, or by such carriers and carriers by railroad, or the maxima or minima, or maxima and minima, to be charged, and the divisions of such rates, fares, or charges as hereinafter provided, and the terms and conditions under which such through routes shall be operated. In the case of a through route, where one of the carriers is a common carrier by water, the Commission shall prescribe such reasonable differentials as it may find to be justified between all-rail rates and the joint rates in connection with such common carrier by water. . . .”
  There was the following colloquy during the debate in the Senate between Senator Wheeler who was in charge of the bill and Senator Norris (84 Cong. Rec. 5874):
  “Mr. NORRIS. Suppose, however, this bill should become a law, and an article of freight were to be transported which admittedly could be carried by water better than by railroad, and suppose the railroad should undertake to reduce its rate down to the water rate: Would there be anything in the law to prevent the railroad from doing that?
  “Mr. WHEELER. Of course there would be.
  “Mr. NORRIS. Disastrous rates can be brought about by reducing them just as well as by increasing them, as a matter of fact.
  “Mr. WHEELER. Exactly.
  “Mr. NORRIS. And the water carrier ought to be protected on the freight that ought to be carried by water rather than on the railroad.
  “Mr. WHEELER. It would be absolutely protected under this measure.
  “Mr. NORRIS. That is what I want to see done.
  “Mr. WHEELER. There is not any question about it. Under the rate-making provision and under the other provisions I have read, there is not any question about it.”
  Id., 5877.
  Id., 5875.
  Note 4, supra.
  The Division of the Commission thought that regard for the “inherent advantages” of water transportation did not require it “to force rail carriers, where they maintain a depressed rate to meet water competition, to further depress their earnings on the traffic in order to favor that same form of competition.” 287 I. C. C., at 407, But part of that critical “depressed rate” — and the one most relevant here — is the $2.26 of the joint all-rail rate received by the East St. Louis-Danville rail carrier. For this same haul, the East St. Louis-Danville rail carrier receives $2.95 on the barge-rail rate. Yet on that haul the railroad has no competition from the water carriers.
choices: attorney general of the United States, or his office specified state board or department of education city, town, township, village, or borough government or governmental unit state commission, board, committee, or authority county government or county governmental unit, except school district court or judicial district state department or agency governmental employee or job applicant female governmental employee or job applicant minority governmental employee or job applicant minority female governmental employee or job applicant not listed among agencies in the first Administrative Action variable retired or former governmental employee U.S. House of Representatives interstate compact judge state legislature, house, or committee local governmental unit other than a county, city, town, township, village, or borough governmental official, or an official of an agency established under an interstate compact state or U.S. supreme court local school district or board of education U.S. Senate U.S. senator foreign nation or instrumentality state or local governmental taxpayer, or executor of the estate of state college or university United States State person accused, indicted, or suspected of crime advertising business or agency agent, fiduciary, trustee, or executor airplane manufacturer, or manufacturer of parts of airplanes airline distributor, importer, or exporter of alcoholic beverages alien, person subject to a denaturalization proceeding, or one whose citizenship is revoked American Medical Association National Railroad Passenger Corp. amusement establishment, or recreational facility arrested person, or pretrial detainee attorney, or person acting as such;includes bar applicant or law student, or law firm or bar association author, copyright holder bank, savings and loan, credit union, investment company bankrupt person or business, or business in reorganization establishment serving liquor by the glass, or package liquor store water transportation, stevedore bookstore, newsstand, printer, bindery, purveyor or distributor of books or magazines brewery, distillery broker, stock exchange, investment or securities firm construction industry bus or motorized passenger transportation vehicle business, corporation buyer, purchaser cable TV car dealer person convicted of crime tangible property, other than real estate, including contraband chemical company child, children, including adopted or illegitimate religious organization, institution, or person private club or facility coal company or coal mine operator computer business or manufacturer, hardware or software consumer, consumer organization creditor, including institution appearing as such; e.g., a finance company person allegedly criminally insane or mentally incompetent to stand trial defendant debtor real estate developer disabled person or disability benefit claimant distributor person subject to selective service, including conscientious objector drug manufacturer druggist, pharmacist, pharmacy employee, or job applicant, including beneficiaries of employer-employee trust agreement, employee health and welfare fund, or multi-employer pension plan electric equipment manufacturer electric or hydroelectric power utility, power cooperative, or gas and electric company eleemosynary institution or person environmental organization employer. If employer's relations with employees are governed by the nature of the employer's business (e.g., railroad, boat), rather than labor law generally, the more specific designation is used in place of Employer. farmer, farm worker, or farm organization father female employee or job applicant female movie, play, pictorial representation, theatrical production, actor, or exhibitor or distributor of fisherman or fishing company food, meat packing, or processing company, stockyard foreign (non-American) nongovernmental entity franchiser franchisee lesbian, gay, bisexual, transexual person or organization person who guarantees another's obligations handicapped individual, or organization of devoted to health organization or person, nursing home, medical clinic or laboratory, chiropractor heir, or beneficiary, or person so claiming to be hospital, medical center husband, or ex-husband involuntarily committed mental patient Indian, including Indian tribe or nation insurance company, or surety inventor, patent assigner, trademark owner or holder investor injured person or legal entity, nonphysically and non-employment related juvenile government contractor holder of a license or permit, or applicant therefor magazine male medical or Medicaid claimant medical supply or manufacturing co. racial or ethnic minority employee or job applicant minority female employee or job applicant manufacturer management, executive officer, or director, of business entity military personnel, or dependent of, including reservist mining company or miner, excluding coal, oil, or pipeline company mother auto manufacturer newspaper, newsletter, journal of opinion, news service radio and television network, except cable tv nonprofit organization or business nonresident nuclear power plant or facility owner, landlord, or claimant to ownership, fee interest, or possession of land as well as chattels shareholders to whom a tender offer is made tender offer oil company, or natural gas producer elderly person, or organization dedicated to the elderly out of state noncriminal defendant political action committee parent or parents parking lot or service patient of a health professional telephone, telecommunications, or telegraph company physician, MD or DO, dentist, or medical society public interest organization physically injured person, including wrongful death, who is not an employee pipe line company package, luggage, container political candidate, activist, committee, party, party member, organization, or elected official indigent, needy, welfare recipient indigent defendant private person prisoner, inmate of penal institution professional organization, business, or person probationer, or parolee protester, demonstrator, picketer or pamphleteer (non-employment related), or non-indigent loiterer public utility publisher, publishing company radio station racial or ethnic minority person or organization protesting racial or ethnic segregation or discrimination racial or ethnic minority student or applicant for admission to an educational institution realtor journalist, columnist, member of the news media resident restaurant, food vendor retarded person, or mental incompetent retired or former employee railroad private school, college, or university seller or vendor shipper, including importer and exporter shopping center, mall spouse, or former spouse stockholder, shareholder, or bondholder retail business or outlet student, or applicant for admission to an educational institution taxpayer or executor of taxpayer's estate, federal only tenant or lessee theater, studio forest products, lumber, or logging company person traveling or wishing to travel abroad, or overseas travel agent trucking company, or motor carrier television station union member unemployed person or unemployment compensation applicant or claimant union, labor organization, or official of veteran voter, prospective voter, elector, or a nonelective official seeking reapportionment or redistricting of legislative districts (POL) wholesale trade wife, or ex-wife witness, or person under subpoena network slave slave-owner bank of the united states timber company u.s. job applicants or employees Army and Air Force Exchange Service Atomic Energy Commission Secretary or administrative unit or personnel of the U.S. Air Force Department or Secretary of Agriculture Alien Property Custodian Secretary or administrative unit or personnel of the U.S. Army Board of Immigration Appeals Bureau of Indian Affairs Bonneville Power Administration Benefits Review Board Civil Aeronautics Board Bureau of the Census Central Intelligence Agency Commodity Futures Trading Commission Department or Secretary of Commerce Comptroller of Currency Consumer Product Safety Commission Civil Rights Commission Civil Service Commission, U.S. Customs Service or Commissioner of Customs Defense Base Closure and REalignment Commission Drug Enforcement Agency Department or Secretary of Defense (and Department or Secretary of War) Department or Secretary of Energy Department or Secretary of the Interior Department of Justice or Attorney General Department or Secretary of State Department or Secretary of Transportation Department or Secretary of Education U.S. Employees' Compensation Commission, or Commissioner Equal Employment Opportunity Commission Environmental Protection Agency or Administrator Federal Aviation Agency or Administration Federal Bureau of Investigation or Director Federal Bureau of Prisons Farm Credit Administration Federal Communications Commission (including a predecessor, Federal Radio Commission) Federal Credit Union Administration Food and Drug Administration Federal Deposit Insurance Corporation Federal Energy Administration Federal Election Commission Federal Energy Regulatory Commission Federal Housing Administration Federal Home Loan Bank Board Federal Labor Relations Authority Federal Maritime Board Federal Maritime Commission Farmers Home Administration Federal Parole Board Federal Power Commission Federal Railroad Administration Federal Reserve Board of Governors Federal Reserve System Federal Savings and Loan Insurance Corporation Federal Trade Commission Federal Works Administration, or Administrator General Accounting Office Comptroller General General Services Administration Department or Secretary of Health, Education and Welfare Department or Secretary of Health and Human Services Department or Secretary of Housing and Urban Development Interstate Commerce Commission Indian Claims Commission Immigration and Naturalization Service, or Director of, or District Director of, or Immigration and Naturalization Enforcement Internal Revenue Service, Collector, Commissioner, or District Director of Information Security Oversight Office Department or Secretary of Labor Loyalty Review Board Legal Services Corporation Merit Systems Protection Board Multistate Tax Commission National Aeronautics and Space Administration Secretary or administrative unit of the U.S. Navy National Credit Union Administration National Endowment for the Arts National Enforcement Commission National Highway Traffic Safety Administration National Labor Relations Board, or regional office or officer National Mediation Board National Railroad Adjustment Board Nuclear Regulatory Commission National Security Agency Office of Economic Opportunity Office of Management and Budget Office of Price Administration, or Price Administrator Office of Personnel Management Occupational Safety and Health Administration Occupational Safety and Health Review Commission Office of Workers' Compensation Programs Patent Office, or Commissioner of, or Board of Appeals of Pay Board (established under the Economic Stabilization Act of 1970) Pension Benefit Guaranty Corporation U.S. Public Health Service Postal Rate Commission Provider Reimbursement Review Board Renegotiation Board Railroad Adjustment Board Railroad Retirement Board Subversive Activities Control Board Small Business Administration Securities and Exchange Commission Social Security Administration or Commissioner Selective Service System Department or Secretary of the Treasury Tennessee Valley Authority United States Forest Service United States Parole Commission Postal Service and Post Office, or Postmaster General, or Postmaster United States Sentencing Commission Veterans' Administration War Production Board Wage Stabilization Board General Land Office of Commissioners Transportation Security Administration Surface Transportation Board U.S. Shipping Board Emergency Fleet Corp. Reconstruction Finance Corp. Department or Secretary of Homeland Security Unidentifiable International Entity