I modified this with my old 9v motor under the coal car (most natural looking) and it works ok but seems to have trouble with more weight. I added a m.o.c. Passenger car and it struggles. Going regular speed at top speed on the regulator, you can hear it whining. Also tried putting in the engine car, worked a little better but same result with it struggling. Any suggestions? Thought about getting another train set with power functions to complement?