Here’s a stab I took years ago at the simplest possible electronic flasher. I tacked one together; and it seemed to work fine. I don’t have any way to show you a schematic; but I’ll try to describe the circuit. I will put Radio Shack stock numbers and prices in parentheses.
Get a bridge rectifier (276-1152, $1.49). Connect one of the two terminals on each side of the “+” marking to the accessory voltage of the transformer or to the center rail of the track. Connect the rectifier terminal on the other side of the “+” to an insulated running rail.
Connect the “+” terminal of the rectifier to the positive terminal of a 1000-microfarad capacitor (272-1019, 2 for $1.69, or 272-1047, 2 for $2.59) and the rectifier terminal opposite the “+” to the negative terminal of the capacitor. Call the positive capacitor terminal V+ and the negative V- from here on.
Connect the crossing-signal common, terminal 1, to V+. Connect the emitters, marked “E”, of two 2N3904 transistors (276-2016, $.69) to V-. Connect one crossing-signal lamp, terminal 2, to the collector, marked “C” of one transistor, the other lamp, terminal 3, to the collector of the other transistor.
Connect a 3300-ohm resistor (271-1122, 5 for $.99) between V+ and the base, marked “B”, of one transistor. Do the same with another resistor for the other transistor. Connect the positive terminal of another 1000-microfarad capacitor (272-1019, 2 for $1.69, or 272-1047, 2 for $2.59) to the collector of one transistor and the negative terminal to the base of the other transistor. Then do the same, switching the roles of the two transistors.
If the circuit flashes too slowly for you, decrease the capacitance of the last two capacitors described; increase capacitance to slow it down. An easy way to cut the speed in half is to put another 1000-microfarad capacitor in parallel with each of the existing capacitors