I use inrush current limiting (thermistor) devices picked up from DigiKey. These are CL-160, P/N KC016L-ND.
The thermistor is simply wired in series with the two motors. Blue wire from one motor connects to the thermistor, other side of the thermistor connects to the yellow wire from the other motor.
The thermistor is a resistor whose resistance varies with temperature. As it gets warmer, the resistance decreases. It can be used to limit the initial flow of current and voltage for a gradual start. On the GP30, I used only one. On my older engines I use two. The GP30 is not quite the racehorse that older Williams locos are, and I think two thermistors might take "too much off the top", but I did not experiment to confirm.
These are 2.8 Amps, you can find them at higher current levels if needed. Resistance varies from 5.0 Ohms at room temp down to 0.2 Ohms when warm.
-- Ken
