The idea is simple. If you boil water in a closed pot, the steam takes up a lot more space than the water did. If you let that steam push a piston - a heavy rod inside a tube - the piston moves. Connect that piston to wheels, and the wheels turn. That is how a steam train works.
Before steam, people travelled by horse, by sailing ship, or on foot. A journey from London to Edinburgh could take more than a week. With steam trains, the same trip took less than a day. Suddenly fresh food, newspapers and post could travel hundreds of kilometres overnight.
The most famous British steam train is called the Mallard. In 1938 it set the world speed record for a steam locomotive - 203 km/h, or about 126 miles per hour. That record has never been beaten by another steam train.
Steam engines also ran factories, mills, ships and the world's first underground railway. Almost every modern engine - in cars, planes and even rockets - is based on the same basic idea that Watt and Stephenson worked out: heat something up, let it expand, and capture the push.