How many volts to smoke unit motor?

I would guess 5 vdc 

I would disconnect the fan motor and see if it will start and run on 2 volts dc if not it is  bad and you need a new motor. sometimes the motor is stuck and just need one tiny drop of oil on the bearing next to the impeller! do you have power supply to test the motor!

The motors are 5V motors, I test them with 1.5V, you can do that with a battery.  If they don't run on 1.5V, I consider them bad.  Of course, for more sophisticated testing, measuring the current is also good, at 5VDC, a good smoke motor should not be using more than 30-40ma.

My first year Polar Express died, and I sent it back for repairs.  It ran fine for months, but then the smoke fan quit.   This time I did what Alan suggested using a AA battery. After a SMALL drop of oil, it ran again and has been working ever since.  Good luck!  Since your board went out, are you planning on running on straight DC, or using an electronic reverse unit for the motor or....? 

Thanks everyone for your helpful comments about voltage to my S-Gauge smoke unit motor.  I was going to do a "Rube Goldberg" straight DC hookup to the motor, as there are issues with the board (per expert advice from Gunrunner).  All tests have shown that the motor is good, and it is a new motor.  I put it on the track one more time, turned the smoke switch back on, and it smokes like crazy again.  Sure is a demon in this engine!

  If the maximum voltage on the fan is 5 v Dc  ,what is the maximum voltage on the resistors ?

You won't want to put more than about 6-7 volts or so on that 8 ohm smoke unit.and 5 volts max on fan motor!

That's a somewhat complex question.  The resistors are driven by a chopped waveform, the maximum RMS value is probably in the 7-8 VRMS I'd imagine, perhaps slightly lower.