"I have a S scale locomotive that has a 3 amp stall current (approximately 4 ohms resistance). Normal running current of the loco is 0.6 amps. If I add a 2 ohm resistor in series with it, stall current would be limited to 2 amps allowing the use of a HO scale decoder (with BEMF). I recognize the top speed of the loco would be reduced but the decoder BEMF would still give good speed control. Also, the resistor needs to be 10 watt (2A X 2A X 2 ohm).
This seems like a less expensive way to go than using a larger more expensive O gauge decoder. Are there any flaws in this logic?" Answer: Using Resistor(s) to limit the stall (peak) current of the motor might work as long as you reduce the motor current to working current range of the decoder. I would use T1 as being the most robust of all our decoders. There is something that you need to be a very careful with - if you lower the motor current too much it might not have enough power to run the locomotive with a load. The performance of the BEMF should not be affected by the use of resistors.