Steam Locomotives Required Up to 14 Coupled Driving Wheels

The evolution of steam locomotive design saw dramatic increases in power and complexity. George and Robert Stephenson's Rocket of 1829, which won the Rainhill Trials, had only a single pair of driving wheels. However, as railways needed to haul heavier freight and passenger trains, locomotive designers added more driving wheels. Within a few decades, four coupled wheels became common. Eventually, some of the most powerful steam locomotives ever built had as many as 14 coupled driving wheels. These massive machines were necessary to distribute the locomotive's weight and increase traction for pulling heavy loads. Driving wheel sizes also varied by purpose, with passenger engines typically having larger wheels between 72 to 80 inches in diameter for higher speeds, while freight engines used smaller wheels between 54 to 66 inches for greater pulling power. The multitube boiler and steam blast exhaust system pioneered on the Rocket remained fundamental features of steam locomotive design until the end of the steam era.