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A 6-4-4-6 steam locomotive, in the Whyte notation for describing locomotive wheel arrangements, is one with six leading wheels in a leading truck, two sets of four driving wheels, and six trailing wheels in a trailing truck.
Only one locomotive was produced to this arrangement, the Pennsylvania Railroad's sole class S1 of 1939. This experimental locomotive was exhibited at the 1939 New York World's Fair, and was afterward placed in limited service between Chicago, Illinois and Crestline, Ohio. The locomotive was too large to work elsewhere in the system. It was capable of very high speeds, although no documentary evidence has so far surfaced to add credence to stories of record-breaking performance.
| Steam locomotive types
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| Single engine types
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| 0-2-2, 2-2-0, 2-2-2, 2-2-4, 4-2-0, 4-2-2, 4-2-4, 6-2-0
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| 0-4-0, 0-4-2, 0-4-4, 2-4-0, 2-4-2, 2-4-4, 4-4-0, 4-4-2, 4-4-4
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| 0-6-0, 0-6-2, 0-6-4, 2-6-0, 2-6-2, 2-6-4, 4-6-0, 4-6-2, 4-6-4
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| 0-8-0, 0-8-2, 2-8-0, 2-8-2, 2-8-4, 4-8-0, 4-8-2, 4-8-4, 6-8-6
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| 0-10-0, 0-10-2, 2-10-0, 2-10-2, 2-10-4, 4-10-0, 4-10-2
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| 0-12-0, 2-12-2, 4-12-2
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| Duplex engine types
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| 4-4-4-4, 6-4-4-6, 4-4-6-4, 4-6-4-4
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| Mallet (articulated) types
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| 0-4-4-0, 2-4-4-2
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| 0-6-6-0, 2-6-6-2, 2-6-6-4, 2-6-6-6, 4-6-6-4
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| 0-8-8-0, 2-8-8-0, 2-8-8-2, 2-8-8-4, 4-8-8-2, 4-8-8-4
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| 2-10-10-2, 2-8-8-8-2, 2-8-8-8-4
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