@Steve Tyler posted:...
I did find a two-pack of 12 volt relays on Amazon: https://smile.amazon.com/Ulinc...ps%2C111&sr=8-28 Do you think this might work? It's a 12 VDC relay, but I see no reason why AC at track voltage (especially in series with a flasher bulb, as Stan's schematic shows) wouldn't work just as well, and the inclusion of a socket and leads with the relays would make installation a lot easier....
Applying 12V AC to a 12V DC-coil relay will chatter/buzz the relay contacts as the AC voltage switches polarity.
12V AC-coil relays are out there but harder to find. Yeah, if you use the search term "12V AC relay" on Amazon, it thinks you're looking for an automotive Air Conditioner (AC) relay and you just go around in circles! 
Here's a "genuine" 12V AC-coil relay on eBay, R10-5A10-12, but widely available such as DigiKey...and even available thru Walmart of all places! 
You can socket it but that's another component.
Hmm. The math is a little more involved than simply using coil resistance. That is, resistance as measured by a multimeter is relevant for DC operation. The datasheet for the AC coil relay doesn't even tell you the coil resistance and instead specifies 2VA (Volt-Amps) as coil power. Cutting to the chase, the AC coil current when applying 12V AC would be 167mA (= 2VA / 12VAC). It's that coil winding that mucks things up; in techno-speak we're dealing with Impedance instead of Resistance. Gets pretty nerdy. OK, hold that thought.
Back to your 12V DC-coil Amazon relay. I did not see the coil resistance either but found what appears to be a similar automotive relay that actually provides some useful parameters such as 80 Ohms coil resistance:
So applying 12V DC to the 80 Ohm coil, the DC current should be 150 mA (= 12V / 80 Ohms).
OK, so tying together the technical tidbits, the good news is we're at least on the same page as the operating currents are similar... albeit apples and oranges (i.e., AC vs. DC). I'll address that later if anyone is still awake. 
In any case, if trying to design the circuit, choose components, etc., we need to know the parameters of the flashing bulb. I actually scrounged around and found some old incandescent xmas strings but no flashing bulbs. However I found some bulbs I had extracted from a toy truck powered by 2 D-cells (i.e., 3V) that had flashing and fixed lights. I'll use this as a proxy for your xmas flasher bulb.
I can see now that trying to "crack open" a flasher bulb to fiddle with the bimetallic element would be a fool's errand. Anyway, I got some DC measurements. At 2V DC, current when on was 240mA (or 8.3 Ohms). At 3V DC, current when on was 320mA (or 9.4 Ohms). It would not flash below ~2V; I did not run it much more than 3V so as not to burn it out. The key is one can get "useful" data with a meter measuring DC parameters because a bulb is not a coil. The "fixed" bulb that are about the same brightness measured 120mA at 3V DC (or 25 Ohms).
So I see (at least) 2 directions you can take:
1. Stick with a "genuine" AC-coil relay. Make some measurements on your flasher and fixed lamp bulbs with a meter to get their DC parameters. Then do the math. I think the numbers are in the ballpark such that you can balance the voltage split between the AC-coil relay and the flasher bulb by placing a few fixed-bulbs across the AC-coil...but I'm getting ahead of myself. At $10+ for just the relay, I think this approach is kind of spendy. But it would be "old-school" as a relay might be age-appropriate for a Marx era layout.
2. Try to get a DC-coil relay to work in an AC environment. This is what I'd do as 12V DC-coil relays are widely available and inexpensive. However, as I see it, you would need some 10-cent diodes which presumably violates the old-school approach?
