Randall Styx says:

I’m not sure if “relay” is what you’re looking for. A relay is a type of switch. Perhaps the most common application is to use a low voltage or low amperage switch to activate a high voltage or amperage switch. This enables one to have a small switch and small wires close to the operator instead of having to run large wires long distances and through crowded spaces (like inside an automobile dashboard). It also makes it safer for the operator. A low powered circuit activates an electromagnet in the relay that opens or closes the contacts of a high powered switch.

Vintage Lionel O gauge trains had a type of relay that operated mostly automatically. Each time current was supplied to the locomotive, the relay engaged a type of switch that would reverse the polarity of the motor circuit, with a neutral resting space between each. Each time the current was cut, the electromagnet would release and when the current came back on it would pull the switch to the next position, cycling through “forward”, “rest”, “reverse”, “rest” and so on. Those loco’s commonly had another manual switch that would shunt the current away from the relay so that the loco would continue in whatever direction it was going every time the current came back on.

Any relay should have at least four contacts – two for the low power circuit of the electromagnet and two for the high powered circuit switched on and off by the relay. Or, if it is a polarity reversing relay, there will be two contacts to supply power to the electromagnet, and four contacts to control the machine or motor, two of them coming from the power source and two going to the load. One of the “power source” leads could be grounded, but with a polarity reversing switch the load leads cannot be grounded. Current must go to the load and back through the switch before it may go to ground. Usually one can find which two contacts supply the electromagnet by using an ohmmeter or a low powered continuity checker. The electromagnet contacts are often smaller than the load leads. The power in an ohmmeter is seldom strong enough to activate the relay. Once you find the two contacts that allow current to flow you have found the contacts for the electromagnet.(But if it’s a polarity reversing relay, you might have found two of the main contacts.) By supplying the required current to those contacts you will activate the relay. The other contacts are for the ultimate load, which the relay controls.

This refers to simple relays. They can be combined in rather intricate ways inside a “black box”, but gives the basic idea. Some relays have two electromagnet circuits – one to turn the main switch on, the other to turn it off. These are activated by momentary switches so that current does not flow through the relay all the time, heating it up and using up electricity. In other cases a single electromagnet can do both by reversing the polarity to the electromagnet. Some relays operate without electromagnets, using heat. In each case the purpose is the same: to relay the action of a convenient or safe switch to an inconvenient or potentially dangerous switch. The safe and convenient switch can be operated manually by a person or automatically by a machine or computer program.

How a relay might be used individually to control multiple loco’s all on the same power source I don’t know. Perhaps a relay controlled by a battery and a radio operated switch could control the power coming from the track to the motor(s) inside the loco. An electromagnet embedded in the track between ties could activate a magnetically controlled switch on the bottom of the loco. When you want the train to stop, you activate the magnet to turn off the switch on the loco when it passes the location of the electromagnet so it comes to a stop even as other trains on the same power source continue running. A low voltage DC circuit through the track and wheels of an AC powered train could activate a magnet to turn off loco’s entering an occupied block. (Some diodes would be needed to separate the blocks.) In either case, one would need to be able to know where the loco’s switch will be once it is stopped so another electromagnet can turn it back on again. Or the loco’s switch could be one that comes back on by itself after a set amount of time.

Hope this doesn’t muddy the waters even more.

Jim Allen says:

Howdy, For what it’s worth… Years ago Tru Scale sold relays that would control train movements and stops and signals as well. I ran a 4×8 layout with 3 of those relays. They were 50 ohms each and were put in series with the track leads. The main loop consisted of 4 blocks. With the 3 relays I was able to control 4 blocks. I could run two trains at the same time without fear of one running into the rear of the other. There were two sets of leaf switches on the relays, one for train control and the other for signals. It was quite a system but all very logical as far as wiring went. I even hooked up a crossing signal to one of the relays so that when a train came by it flashed the signals alternately with a system that I rigged up using a slow speed motor (60 rpm) with an eccentric block of wood on the output shaft and a micro switch with spdt contacts. Being able to adjust the micro switch I could make the lights flash very realistically. I was really tickled with the results as all the other crossing signals I had seen just didn’t give the right results as far as I was concerned. Just to give you an idea how long ago it was…. I ran two trains, one passenger and one freight with rubber band drive Athern F7’s. In order to get them to run slower than as bought, I rigged a countershaft on each side of the motor with a large pulley on the end drive shaft and using a small rubber band (the kind dentists use for retainers). The reduction in speed made them run very well. The one drawback to the system was that when the trains hit a controlled block that was RED the train stopped abruptly, like dead stop. A later engine with flywheels didn’t fair that much better so I rearranged one block so the engine would stop in the tunnel and another one behind a mound so it wasn’t as obvious. I set everything up so my son, who was only 4, could operate it without help. I no longer have that layout but I did start another one just like it but never finished it as it hangs in my shed. The track plan came from a Boy’s Life magazine from the late 30’s. I consisted of a sort of figure 8 with an over/under crossover. There were 7 switches with 3 sidings and a place to reverse direction of the trains. There was a problem with that though as the relays and blocks were set for one direction only. The plans also called for a 1 foot x 8 foot extension in the rear for a switching yard complete with turntable. I have tried to find a copy of that plan over the years without success. There was a lot of switching possibilities within a small HO layout that made it enjoyable to run.