avr_fred:
+1 for using a Teensy 3.5. The cost difference is minimal between a 3.2 and the 3.5, you maintain 5 volt tolerant digital inputs (lost with the LC), the built-in micro SD card which has its own, dedicated SPI CS pin and it supports hardware floating point. Painless to use, fast as blazes and rock solid.
That is true. The 180 mHz is very noticeable...I built a lightsaber with neopixels and the teensy's processing time of that library was night and day over an 16mHz pro micro. The pro micro was just sluggish. It also helps with polyphonic audio I think...which is useful since playing different sounds on the loco (compressors, drain cocks, whistle, chuffing, etc.) making it a bit more realistic.
But yes, the 5v tolerant pins are nice...that way if something shorts with the dc/dc converter (which I think I forgot to mention, but am not worried about) then the board is fine.
I was thinking for the motor controller I could just use four of these: N-Channel MOSFET 60V 30A - COM-10213 - SparkFun Electronics in an h-bridge config like this: https://i.stack.imgur.com/s9Uvc.png
Each one can dissipate 75w with an operating temp range of -55 to +175C. From the sound of it, that would be fine, and four of those guys would take up less space than the MD10C.