4-8-8-4
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In the Whyte notation for the classification of steam locomotives by wheel arrangement, a 4-8-8-4 is a locomotive with a four-wheel leading truck, two sets of eight driving wheels, and a four-wheel trailing truck.
The equivalent UIC classification is (2'D)D2'.
The only significant 4-8-8-4 locomotives built were the Union Pacific 4000 "Big Boy" Class. The UP Big Boys were an expansion of the 4-6-6-4 "Challenger" type articulated locomotive. Large diameter driving wheels were fitted to allow higher speed freight runs in graded terrain. Adding two more sets of driving wheels increased the pulling power of the locomotive and eliminated the need for helper locomotives over steep grades.
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| Steam locomotive types |
| Single engine types |
| 0-2-2 | 2-2-0 | 2-2-2 | 2-2-4 | 4-2-0 | 4-2-2 | 4-2-4 | 6-2-0 |
| 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 |
| 0-6-0 | 0-6-2 | 0-6-4 | 2-6-0 | 2-6-2 | 2-6-4 | 2-6-6 | 4-6-0 | 4-6-2 | 4-6-4 |
| 0-8-0 | 0-8-2 | 0-8-4 | 2-8-0 | 2-8-2 | 2-8-4 | 2-8-6 | 4-8-0 | 4-8-2 | 4-8-4 | 4-8-6 | 6-8-6 |
| 0-10-0 | 0-10-2 | 2-10-0 | 2-10-2 | 2-10-4 | 4-10-0 | 4-10-2 |
| 0-12-0 | 2-12-0 | 2-12-2 | 2-12-4 | 4-12-2 | 4-14-4 |
| Duplex engine types |
| 4-4-4-4 | 6-4-4-6 | 4-4-6-4 | 4-6-4-4 |
| Garratt (articulated) types |
| 0-4-0+0-4-0 | 2-6-0+0-6-2 | 4-6-2+2-6-4 | 2-8-0+0-8-2 | 4-8-4+4-8-4 |
| Mallet (articulated) types |
| 0-4-4-0 | 0-4-4-2 | 2-4-4-2 |
| 0-6-6-0 | 2-6-6-0 | 2-6-6-2 | 2-6-6-4 | 2-6-6-6 | 2-6-8-0 | 4-6-6-2 | 4-6-6-4 |
| 0-8-8-0 | 2-8-8-0 | 2-8-8-2 | 2-8-8-4 | 4-8-8-2 | 4-8-8-4 |
| 2-10-10-2 | 2-8-8-8-2 | 2-8-8-8-4 |
