haven't seen a train for quite a while

Side Rod locomotive

Most railroads operating steam engines you have seen have probably had Side Rod driven engines. These engines were the most common on railroads, but what if you had a specialty railroad, lets say you wanted one to haul loads at fairly slow "even" speeds. Or you had a logging railroad with temporary trackage running through the hills and valleys of the woods or mountains. Side rod engines have an occelation to them a force counter force issue because of the drive rods thrust, they cause a back and forth motion that on a steep grade can cause an engine to lose grip and make the wheels spin. the answer to this was Geared locomotives, three types were designed. 

1. The Climax with it's pistons angled back driving a shaft which drives a fore and aft mounted drive shaft.
2. The Heisler with 45 degree inward facing pistons turning a fore and aft drive shaft.
3. The Shay which had vertical pistons driving a crankshaft and drive shafts along one side of the engine. Giving the unit an off side look when seen from the front.

Heisler geared locomotive

Heisler geared locomotive

Climax geared Locomotive

Shay's drive

Shay geared Locomotive

the gear drive allowed for a smooth steady drive up the grades.