MorganS:
For a motor controller, the VNH7100 can handle up to 18A max and it's tiny. It's also very low RdsON so it won't heat up much. I've never used one before but I just got a bunch in the mail to try out for my next design. At 100milli-ohms it's not going to generate a lot of heat.
The problem is that the amps go cubed... 13A at 100 mOhm = 13x13x0.1 = 16.9W dissipation. That's a lot of heat to handle inside a small train body.
I was calculating at 20-25 mOhm RDS(ON) which some MOSFETs can do (but probably not the p-channel ones).