You can run MTH and Lionel locomotives on each others' track, no problem; it's all the same gauge. The focus of the issue is what type of equipment you plan to run.
What brands of locomotives do you have, and do you know what electronics they have? If you don't know, product numbers would be helpful.
Are you running with just a regular transformer, with Lionchief remote control, the MTH infrared remote or DCS lite handheld (the one that comes in starter sets)?
