Geared steam locomotive
A geared steam locomotive is a type of steam locomotive which uses gearing, usually reduction gearing, in the drivetrain, as opposed to the common directly driven design.
Jackshaft (locomotive)
In a conventional steam locomotive, the crankshaft is one of the driving axles. In a jackshaft-driven steam locomotive, the crankshaft turns a jackshaft which, in turn, turns the driver. Some steam locomotives have had designs intermediate between these extremes, with crankshafts distinct from the driving axle. Phineas Davis's first B&O Grasshopper tested on the B&O in 1831 was in this class, as was the Stockton and Darlington Railway's "Swift" from 1836, where the crankshaft was directly between the driving axles. Both of these examples used vertical cylinders, with the crankshaft in the plane of the driving axles. The former used a geared drive to the first driving axle, the latter used side rods for this linkage. In the latter case, the reason inferred for using a crankshaft distinct from the driven axles was "to take the shocks of working away from the power shaft."