I believe steam is a potential energy when it's contained or under control. When it acts upon a turbine and starts to spin it, the potential energy becomes kinetic energy.
In a 1860's locomotive, I'm guessing there was potential energy in the coal. It was burned to create heat (maybe that would be called thermal energy?) which heats the boiler to create steam. The steam would then be potential energy (able to do work but not currently doing any). Then the steam was released against a piston where it expanded and pushed the piston. The moving piston would then be kinetic energy. As it turns the wheels and moves the train, the whole moving train is also a form of kinetic energy.
When I think of thermal energy, I think of stuff like a cone-shaped greenhouse with a turbine in the center of the cone. When the air inside the greenhouse warms up, it rises and is pushed through the turbine spinning the blades and causing power to be made. And yet at night, when the temperatures cool, the air gets funnels back down to spin the generator yet again.
A Stirling Engine using solar power would be another example...
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