Geared steam locomotive
This gearing is part of the machinery within the locomotive and should not be confused with the pinion that propels a rack locomotive along the rack between the rails. The geared steam locomotives that have been built have been for conventional track, relying on the adhesion between wheels and rail.
Steam locomotive
Locomotives with total adhesion, where all of the wheels are coupled together, generally lack stability at speed. To counter this, locomotives often fit unpowered carrying wheels mounted on two-wheeled trucks or four-wheeled bogies centred by springs/inverted rockers/geared rollers that help to guide the locomotive through curves. These usually take on weight – of the cylinders at the front or the firebox at the rear — when the width exceeds that of the mainframes. Locomotives with multiple coupled-wheels on a rigid chassis would have unacceptable flange forces on tight curves giving excessive flange and rail wear, track spreading and wheel climb derailments. One solution was to remove or thin the flanges on an axle. More common was to give axles end-play and use lateral motion control with spring or inclined-plane gravity devices.