<?xml version="1.0"?>

<ownershipDocument>

    <documentType>4</documentType>

    <periodOfReport>2003-06-11</periodOfReport>


    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001199999</rptOwnerCik>
            <rptOwnerName>DOLAN JAMES V</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>Vice Chairman</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeSecurity>
        <securityTitle>
            <value>Common Stock</value>
        </securityTitle>
        <transactionDate>
            <value>2003-06-11</value>
        </transactionDate>
        <transactionCoding>
            <transactionFormType>4</transactionFormType>
            <transactionCode>I</transactionCode>
            <equitySwapInvolved>0</equitySwapInvolved>
        </transactionCoding>
        <transactionTimeliness>
            <value></value>
        </transactionTimeliness>
        <transactionAmounts>
            <transactionShares>
                <value>3168.5164</value>
            </transactionShares>
            <transactionValue>
                <value>60.85</value>
            </transactionValue>
            <transactionAcquiredDisposedCode>
                <value>D</value>
            </transactionAcquiredDisposedCode>
        </transactionAmounts>
        <postTransactionAmounts>
            <sharesOwnedFollowingTransaction>
                <value>107.9806</value>
                <footnoteId id="F1"/>
            </sharesOwnedFollowingTransaction>
        </postTransactionAmounts>
        <ownershipNature>
            <directOrIndirectOwnership>
                <value>I</value>
            </directOrIndirectOwnership>
            <natureOfOwnership>
                <value>by Managed Account</value>
            </natureOfOwnership>
        </ownershipNature>
    </nonDerivativeSecurity>

    <footnotes>
        <footnote id="F1">Includes holdings in Union Pacific's Payroll Based Stock Ownership Plan as of the Transaction Date.  The reporting person also has the following holdings: 1) 650 shares held indirectly by the reporting person's spouse, and the reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein; and 2) 102,458 shares held directly by the reporting person.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>James V. Dolan</signatureName>
        <signatureDate>2003-06-16</signatureDate>
    </ownerSignature>

</ownershipDocument>
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4
2003-06-11
0
0000100885
UNION PACIFIC CORP
UNP
0001199999
DOLAN JAMES V
0
1
0
0
Vice Chairman
Common Stock
2003-06-11
4
I
0
3168.5164
60.85
D
107.9806
I
by Managed Account
Includes holdings in Union Pacific's Payroll Based Stock Ownership Plan as of the Transaction Date.  The reporting person also has the following holdings: 1) 650 shares held indirectly by the reporting person's spouse, and the reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein; and 2) 102,458 shares held directly by the reporting person.
James V. Dolan
2003-06-16