The 740 is not the best engineered unit. The best you can do is keep it clean and well lubricated. I find that using a power supply with a non sinusoidal output such as the MRC Dual Power O27 helps. The pulse like nature of the output gets the unit running at a lower voltage which slightly reduces the heat build up.
From your description and the other post is sounds like you are operating a SIT steam engine with the reverse unit lever protruding out the top of the engine casting. If it is hot it sounds like the engine needs cleaned but you have already done that. It is possible the brushes could be worn and the springs weak. My diecast steam engines can run for an hour without becoming more than warm. Finally it is possible that running the engines slightly faster will reduce the heat generated in the motor windings.
