This is how my train boy works Trains do need water so when When heated, water turns to an invisible vapor known as steam. The volume of water expands as it turns to steam inside the boiler, creating a high pressure. The expansion of steam pushes the pistons that connect to the driving wheels that operate the locomotive. trains are fueled by burning combustible material (usually coal, oil or – now rare – wood) to heat water in the locomotive's boiler to the point when it becomes gaseous and its volume increases 1700 times. Functionally, it is a steam engine on wheels.