<?xml version="1.0"?>

<ownershipDocument>

    <documentType>4</documentType>

    <periodOfReport>2003-06-12</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>
            <footnoteId id="F1"/>
        </securityTitle>
        <transactionDate>
            <value>2003-01-14</value>
        </transactionDate>
        <transactionCoding>
            <transactionFormType>5</transactionFormType>
            <transactionCode>G</transactionCode>
            <equitySwapInvolved>0</equitySwapInvolved>
        </transactionCoding>
        <transactionTimeliness>
            <value>E</value>
        </transactionTimeliness>
        <transactionAmounts>
            <transactionShares>
                <value>500</value>
            </transactionShares>
            <transactionValue>
                <value>0</value>
            </transactionValue>
            <transactionAcquiredDisposedCode>
                <value>D</value>
            </transactionAcquiredDisposedCode>
        </transactionAmounts>
        <postTransactionAmounts>
            <sharesOwnedFollowingTransaction>
                <value>101958</value>
            </sharesOwnedFollowingTransaction>
        </postTransactionAmounts>
        <ownershipNature>
            <directOrIndirectOwnership>
                <value>D</value>
            </directOrIndirectOwnership>
        </ownershipNature>
    </nonDerivativeSecurity>

    <nonDerivativeSecurity>
        <securityTitle>
            <value>Common Stock</value>
        </securityTitle>
        <transactionDate>
            <value>2003-06-12</value>
        </transactionDate>
        <transactionCoding>
            <transactionFormType>4</transactionFormType>
            <transactionCode>S</transactionCode>
            <equitySwapInvolved>0</equitySwapInvolved>
        </transactionCoding>
        <transactionTimeliness>
            <value></value>
        </transactionTimeliness>
        <transactionAmounts>
            <transactionShares>
                <value>5600</value>
            </transactionShares>
            <transactionValue>
                <value>61.03</value>
            </transactionValue>
            <transactionAcquiredDisposedCode>
                <value>D</value>
            </transactionAcquiredDisposedCode>
        </transactionAmounts>
        <postTransactionAmounts>
            <sharesOwnedFollowingTransaction>
                <value>96358</value>
            </sharesOwnedFollowingTransaction>
        </postTransactionAmounts>
        <ownershipNature>
            <directOrIndirectOwnership>
                <value>D</value>
            </directOrIndirectOwnership>
        </ownershipNature>
    </nonDerivativeSecurity>

    <nonDerivativeSecurity>
        <securityTitle>
            <value>Common Stock</value>
        </securityTitle>
        <transactionDate>
            <value>2003-06-12</value>
        </transactionDate>
        <transactionCoding>
            <transactionFormType>4</transactionFormType>
            <transactionCode>S</transactionCode>
            <equitySwapInvolved>0</equitySwapInvolved>
        </transactionCoding>
        <transactionTimeliness>
            <value></value>
        </transactionTimeliness>
        <transactionAmounts>
            <transactionShares>
                <value>400</value>
            </transactionShares>
            <transactionValue>
                <value>61.13</value>
            </transactionValue>
            <transactionAcquiredDisposedCode>
                <value>D</value>
            </transactionAcquiredDisposedCode>
        </transactionAmounts>
        <postTransactionAmounts>
            <sharesOwnedFollowingTransaction>
                <value>95958</value>
                <footnoteId id="F2"/>
            </sharesOwnedFollowingTransaction>
        </postTransactionAmounts>
        <ownershipNature>
            <directOrIndirectOwnership>
                <value>D</value>
            </directOrIndirectOwnership>
        </ownershipNature>
    </nonDerivativeSecurity>

    <footnotes>
        <footnote id="F1">The reporting person made a charitable contribution to a non-affiliated entity.</footnote>
        <footnote id="F2">The reporting person also has the following indirect holdings: 1) 118.7437 units by a Managed Account which include holdings in Union Pacific's Payroll Based Stock Ownership Plan as of the Transaction Date; and 2) 650 shares held by the reporting person's spouse.  The reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein.</footnote>
    </footnotes>

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

</ownershipDocument>
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4
2003-06-12
0
0000100885
UNION PACIFIC CORP
UNP
0001199999
DOLAN JAMES V
0
1
0
0
Vice Chairman
Common Stock
2003-01-14
5
G
0
E
500
0
D
101958
D
Common Stock
2003-06-12
4
S
0
5600
61.03
D
96358
D
Common Stock
2003-06-12
4
S
0
400
61.13
D
95958
D
The reporting person made a charitable contribution to a non-affiliated entity.
The reporting person also has the following indirect holdings: 1) 118.7437 units by a Managed Account which include holdings in Union Pacific's Payroll Based Stock Ownership Plan as of the Transaction Date; and 2) 650 shares held by the reporting person's spouse.  The reporting person disclaims beneficial ownership of these securities except to the extent of his pecuniary interest therein.
James V. Dolan
2003-06-16