What makes this control unique is its momentum feature, which adds a degree of realism. The circuit will opera
te well for trains that draw up to 1 A at 15 V. None of the components are critical. In the start mode, current source Ql charges capacitor CI. The charge current and start-up time are adjusted by resistor R2. In the stop mode, current-sink Q2 discharges capacitor CI. The discharge current and stop time are set by resistor R4. In the coast mode, op amp Ul draws very little current from CI, so the speed will remain nearly constant for some time, and then gradually decrease. Transistors Q3 and Q4 form a Darlington emitter-follower to amplify the output of Ul. Diode D7 reduces the output by about 0.8 V. Another diode could be added in series to decrease the output to 0 in the stop mode.