Some of my favorite locomotives are those produced by Lima Iron
Works, based on the patents of Ephraim Shay (1839–1916).
These Shay locomotives were the most successful of the various
geared locomotive designs.
The drivetrain was modified from the original patent designs by
Lima Iron Works, mainly to feature shorter gearing. This increased
torque at the wheels, making Shays especially useful for logging
and other fields which required steep ascents. This tied back to
Ephraim Shay's impetus for developing this novel drive system in
the first place: his dealings in the logging industry.
Early diesel switchers & road switchers
I've always had a soft spot for early diesel in North America,
before railroads converged on forms like the GP-38 and so forth.