What is the tom thumb?

It is the name of the first railroad steam engine in the U.S, invented by Peter Cooper in 1830.

So Tom Thumb: First Railroad Steam Engine...

Fun fact:

In a race between the engine and a horse, the horse won. Yet the B&O railroad owners were convinced that steam engines were the engines of the future.

The Tom Thumb was also the name of the small boat in which explorers George Bass and Matthew Flinders underwent extensive exploration south of Sydney, Australia, including the circumnavigation of what was then Van Diemen's Land (now Tasmania).