GWR Mercury Class

The GWR Mercury Class was one of the earliest express locomotive classes to operate on the Great Western Railway, introduced in 1839 as 2-2-2 single-driver broad-gauge engines preceding the definitive Firefly Class of 1840. The Mercury Class was among the first attempts to provide the GWR with purpose-built express locomotives after the railway's difficult opening years with a heterogeneous and often unreliable collection of outside-built engines, and represented Daniel Gooch's early exploration of the locomotive configuration that would produce the Firefly's breakthrough performance.

The 2-2-2 single-driver layout with a large central driving wheel was the established Victorian express configuration, and Gooch applied it to the GWR's broad gauge with driving wheels of 6 ft diameter — generous but smaller than the 7 ft wheels he would specify for the Firefly the following year. The Mercury Class represented a transitional stage in GWR locomotive development: more capable than the mixed fleet that preceded it, but not yet the step change in performance that the Firefly achieved in 1840.

The Mercury Class gave service in the GWR's earliest years and was relatively short-lived, superseded by the Firefly Class whose superior performance quickly rendered the earlier express types obsolete. None was preserved.