<?xml version="1.0"?>

<ownershipDocument>

    <documentType>4</documentType>

    <periodOfReport>2003-05-30</periodOfReport>


    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000100885</issuerCik>
        <issuerName>UNION PACIFIC CORP</issuerName>
        <issuerTradingSymbol>UNP</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001200009</rptOwnerCik>
            <rptOwnerName>KNIGHT ROBERT M JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Senior Vice President Finance</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeSecurity>
        <securityTitle>
            <value>Common Stock</value>
        </securityTitle>
        <transactionDate>
            <value>2003-05-30</value>
        </transactionDate>
        <transactionCoding>
            <transactionFormType>4</transactionFormType>
            <transactionCode>S</transactionCode>
            <equitySwapInvolved>0</equitySwapInvolved>
        </transactionCoding>
        <transactionTimeliness>
            <value></value>
        </transactionTimeliness>
        <transactionAmounts>
            <transactionShares>
                <value>2000</value>
            </transactionShares>
            <transactionValue>
                <value>60</value>
            </transactionValue>
            <transactionAcquiredDisposedCode>
                <value>D</value>
            </transactionAcquiredDisposedCode>
        </transactionAmounts>
        <postTransactionAmounts>
            <sharesOwnedFollowingTransaction>
                <value>27648</value>
                <footnoteId id="F1"/>
            </sharesOwnedFollowingTransaction>
        </postTransactionAmounts>
        <ownershipNature>
            <directOrIndirectOwnership>
                <value>D</value>
            </directOrIndirectOwnership>
        </ownershipNature>
    </nonDerivativeSecurity>

    <footnotes>
        <footnote id="F1">The reporting person also has the following indirect holdings: 1) 1,548.7624 units which represent the conversion of restricted stock to fully vested stock units with a distribution ratio of 1:1 - payable only in shares of common stock at termination of employment or a date certain; and 2) 94.8634 units by a Managed Account which include holdings in employee benefit plans, Tax Reduction and Payroll Based Stock Plans, as of the transaction date.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Robert M. Knight, Jr.</signatureName>
        <signatureDate>2003-06-03</signatureDate>
    </ownerSignature>

</ownershipDocument>
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4
2003-05-30
0
0000100885
UNION PACIFIC CORP
UNP
0001200009
KNIGHT ROBERT M JR
0
1
0
0
Senior Vice President Finance
Common Stock
2003-05-30
4
S
0
2000
60
D
27648
D
The reporting person also has the following indirect holdings: 1) 1,548.7624 units which represent the conversion of restricted stock to fully vested stock units with a distribution ratio of 1:1 - payable only in shares of common stock at termination of employment or a date certain; and 2) 94.8634 units by a Managed Account which include holdings in employee benefit plans, Tax Reduction and Payroll Based Stock Plans, as of the transaction date.
Robert M. Knight, Jr.
2003-06-03