Hi Tim,
I'm making a small single locomotive that my son can ride on. I know the circuit components I picked out are only rated for 15 amps, but I figured if I can get the circuit correct then I could upgrade the components to handle more amps and heat dissipation (heat sinks), like changing the 2N3055 to a 2N3771 which can handle 30 amps etc. The engine or trolley is going to be 3d printed polymer making it much lighter then conventional engines and rolling stock in this gauge. I have already tested a Bettendorf bogie in this gauge and had success. It is able to handle 125lb load so far. So my next step would be a powered bogie probably a single EMD style B-B to house an electric scooter motor from ebay.
As you guessed, true scale in 7.5 gauge isn't a priority to me as much as functionality. As I add more components to this trolley, I plan to measure the amps through the circuit and heat build up from the high power transistors and change components accordingly as loads increase. I'm just not 100% on circuit layout. I'm a mechanical engineer so electrical circuits aren't my forte. I appreciate all the help I could get on this circuit. What I have so far is attached below.
thanks,
Chris