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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 C3la6m3LqAZgkziNyb5pfknMaiyD7z0ppv474/+Ye4gCzT5ILYVNPFYZDS4suk7o
 FOFI4e9qOCArnaz8Gm0BfA==

<SEC-DOCUMENT>0000938885-04-000002.txt : 20040203
<SEC-HEADER>0000938885-04-000002.hdr.sgml : 20040203
<ACCEPTANCE-DATETIME>20040203211208
ACCESSION NUMBER:		0000938885-04-000002
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20040202
FILED AS OF DATE:		20040203

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			ROGERS JOHN W JR
		CENTRAL INDEX KEY:			0000938885

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-02328
		FILM NUMBER:		04564614

	BUSINESS ADDRESS:	
		STREET 1:		ARIEL CAPITAL MANAGEMENT, INC.
		STREET 2:		200 EAST RANDOLPH DRIVE, SUITE 2900
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60601

	MAIL ADDRESS:	
		STREET 1:		ARIEL CAPITAL MANAGEMENT, INC.
		STREET 2:		200 EAST RANDOLPH DRIVE, SUITE 2900
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60601

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			GATX CORP
		CENTRAL INDEX KEY:			0000040211
		STANDARD INDUSTRIAL CLASSIFICATION:	TRANSPORTATION SERVICES [4700]
		IRS NUMBER:				361124040
		STATE OF INCORPORATION:			NY
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		500 W MONROE  ST
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60661
		BUSINESS PHONE:		3126216200

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	GENERAL AMERICAN TRANSPORTATION CORP
		DATE OF NAME CHANGE:	19750722
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc4.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2004-02-02</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000040211</issuerCik>
        <issuerName>GATX CORP</issuerName>
        <issuerTradingSymbol>GMT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000938885</rptOwnerCik>
            <rptOwnerName>ROGERS JOHN W JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>ARIEL CAPITAL MANAGEMENT, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>200 EAST RANDOLPH</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60601</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2004-02-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>991.39</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>24.4605</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12831.01</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Includes that portion of (a) the quarterly payment of the annual directors' fee (582.5719 shares) credited to an account established for each director under the GATX Directors' Phantom Stock Plan (&quot;Phantom Stock Plan&quot;) and (b) the annual retainer and meeting fee (408.8224 shares) which the reporting person elected to invest in units of phantom stock under the Directors' Deferred Fee Plan which in each case will be settled in common stock upon the expiration of the director's service on the Board.</footnote>
        <footnote id="F2">Includes 84.1916 shares acquired pursuant to a dividend reinvestment feature of the Phantom Stock Plan and 32.1470 shares acquire
d pursuant to a dividend reinvestment feature of the Deferred Fee Plan.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Lisa M. Ibarra, by Power of Attorney on behalf of John W. Ro</signatureName>
        <signatureDate>2004-02-03</signatureDate>
    </ownerSignature>
</ownershipDocument>

</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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