BSL Physics Glossary - engine - definition
It makes something move. A car has an engine in it which allows it to move. The same is true of an old-fashioned steam engine. This has a different sort of engine with pistons. When these work, they make the train move. The reason we sign engine like this is because inside an engine there is a piston, which pushes up and down. In a steam engine the piston pushes back and forth.
Picture from Wikimedia