This is a great question! There are actually quite a few kinds of trains, and each kind has a different method of propulsion (moving forward).

The first trains were powered by steam locomotive engines. Essentially, a steam engine works by taking advantage of the fact that when liquid water boils and becomes gas, it expands.

A steam engine consists of a boiler, which carries water and is attached to a cylinder and piston. The water in the boiler is heated, either with wood or coal, and as the water boils, steam is funneled into the cylinder, which causes the piston to move back and forth. The piston in turn is attached to a machine that moves the wheels of the train!

These days, however, there are lots of ways to power trains. Electricity can be used to power train motion, like the cable cars you can find in San Francisco. There are also trains that take advantage of gravity to move forward, and some work by taking advantage of differences in air pressure.

Maglev (magnetic levitation) is another method for propelling trains forward and is often used for monorail trains, which can be found in many big cities all over the world. In this technology, magnets are used to levitate ("push up on") the train so it hovers above the rail. A large electric coil which generates a magnetic field is used to propel the train forward using an alternating current. I hope this helps!