Set 7866 contained a 4x4x2 flash unit for the level crossing lights.
It plugs into one of the ports on the level crossing panel switch.
This port is on all the time when the barriers are down, whereas
the other port, for the barrier motors, has a momentary switch contact.

I recently reverse-engineered the unit to draw the circuit diagram.

The unit contains two transistors, an NPN BC547B and an
NPN darlington MPSA13.
The circuit is an RL relaxation oscillator, using two small chokes.
The high gain of the darlington transistor helps to square-up the output.
The rating of the darlington transistor is 1 Amp, but the diode in series
with it is only a signal diode.  However, the output is only on for half
the time.
Another diode protects the darlington transistor from inductive spikes.
One might have expected capacitors to be used instead of chokes, but
none of these were used.  This is unusual but the chokes are smaller than
the capacitors would be for the flashing frequency required.
The PCB is single-sided.

Circuit Operation:
Begin the cycle when the darlington transistor is on.  The lights turn on.
The turning on of the darlington transistor raises its emitter voltage.
There is about 0.8V C-E across the transistor, and 0.7V across the diode, so
the 18R resistor and the load (light bricks in parallel with 820R) drops the rest
of the 12V supply.  Two 12V light bricks in parallel have a resistance of about
18 ohms, so the voltage at the darlington transistor emitter is about 6V at
this time.  Ths raises the base voltage of the BC547B transistor through the
4M7 resistor, turning it on, which turns the darlington transistor off.

As the darlington transistor turns off, the chokes ramp up in current.
The current flowing through the right choke also flows into the base of the
BC547B transistor, keeping it on.  This keeps the darlington transistor off.
The left choke saturates first, due to the smaller resistance in its path, so
its current becomes constant at the maximum value of about 12uA.
the choke value of 22mH is 22mA/s/V, so the current ramp takes very little time.
The saturation of the left choke robs the other choke of current because of the
limited current that can get through the common 1M resistor.
This reverses the current through the right choke, pulling the BC547B transistor
base low and turning it off.
This in turn switches on the darlington transistor, turning on the lights.
The darlington transistor has a lower resistance path to ground than the 1M
through the chokes, so the current through the left choke is reversed.
The voltage between the darlington transistor collector and ground with the
transistor saturated (on) is about 6.8V, with the base at about 7.2V.
Both chokes reduce in current to a value that matches the voltage across them
and the cycle begins again.

Since the resistance of the light bricks changes the darington emitter voltage
when it is on, the number of light bricks changes the flashing frequency.

Mark Bellis
April 2010