Low Cost Model Train Controller (November 1995):
Excess current can flow through D5 if both VR1 and VR2 are set to
minimum resistance and both the 'Stop' and 'Go' buttons are
pressed simultaneously. In this unlikely situation, D5 can burn
out, causing the controller to lose its 'inertia'. A 1k resister
placed in series with the wiper of VR1 will protect this diode,
while while not interfering with the normal operation of the
controller.