Model live steam locomotives predominate as the most popular modelling subject in model engineering. As such they deserve special mention. Live steam refers to the use of pressurised steam, heated in the model locomotive's own boiler, to turn the wheels via miniature steam cylinders. Not all locomotives are live steam - some model engineers make model locomotives powered by electricity or internal combustion engines. The criteria however is that the model is self-propelled, hence requiring an engine to be made or motor to be installed, as opposed to the (usually much smaller) model trains that rely on an electrified track to run.