<SEC-DOCUMENT>0001562180-24-001045.txt : 20240205
<SEC-HEADER>0001562180-24-001045.hdr.sgml : 20240205
<ACCEPTANCE-DATETIME>20240205164312
ACCESSION NUMBER:		0001562180-24-001045
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20240201
FILED AS OF DATE:		20240205
DATE AS OF CHANGE:		20240205

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Holmes John McClain III
		CENTRAL INDEX KEY:			0001554880
		ORGANIZATION NAME:           	

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-02328
		FILM NUMBER:		24596401

	MAIL ADDRESS:	
		STREET 1:		1100 N. WOOD DALE ROAD
		CITY:			WOOD DALE
		STATE:			IL
		ZIP:			60191

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			GATX CORP
		CENTRAL INDEX KEY:			0000040211
		STANDARD INDUSTRIAL CLASSIFICATION:	TRANSPORTATION SERVICES [4700]
		ORGANIZATION NAME:           	01 Energy & Transportation
		IRS NUMBER:				361124040
		STATE OF INCORPORATION:			NY
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		233 SOUTH WACKER DRIVE
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60606-7147
		BUSINESS PHONE:		3126216200

	MAIL ADDRESS:	
		STREET 1:		233 SOUTH WACKER DRIVE
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60606-7147

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

    <schemaVersion>X0508</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2024-02-01</periodOfReport>

    <notSubjectToSection16>false</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001554880</rptOwnerCik>
            <rptOwnerName>Holmes John McClain III</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1100 N. WOOD DALE RD.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>WOOD DALE</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60191</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>true</isDirector>
            <isOfficer>false</isOfficer>
            <isTenPercentOwner>false</isTenPercentOwner>
            <isOther>false</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>false</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2024-02-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>false</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>11.00</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>124.9839</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>260.00</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents additional shares of RSUs credited to the reporting person's account under the GATX Corporation Directors' Voluntary Deferred Fee Plan Amended and Restated as of January 1, 2022 (the &quot;Deferred Fee Plan&quot;) credited on the transaction date. Each share of RSU represents the right to receive one share of Issuer's common stock upon settlement. The shares of RSUs are generally payable on a deferred basis in common stock at the election of the reporting person upon the reporting person's termination of service on the Issuer's board of directors.</footnote>
        <footnote id="F2">Represents 11 RSUs acquired under the Deferred Fee Plan resulting from the reporting person's election to defer the annual cash retainer and other cash fees payable to the reporting person in the form of RSUs under the Deferred Fee Plan.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Lisa M. Ibarra, by Power of Attorney on behalf of John M. Holmes</signatureName>
        <signatureDate>2024-02-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
