The only thing I don’t like about Marx trains is that most of them don’t have a switch to lock the locomotive in one direction. Fortunately it’s not hard to add a reverse lockout switch for Marx if you don’t want to do a temporary lockout.
On cheaper postwar Marx locomotives with a reverse, you’ll find three wires soldered to the headlight socket. The thinnest of those wires is from the reverse unit. Higher-end Marx locomotives with smoke units have a bus bar near the reverse unit. If you’re modifying one of those, look for the wire on the bus bar that runs through the bar into the reverse unit’s coil. On prewar Marx motors, the wire from the reverse unit is often soldered to a fahnestock clip that contacts the headlight. On this one as well, the wire you want is the one that goes into the reverse unit’s coil.
Desolder that wire, then carefully solder about a six-inch length of wire to it and insulate the splice with a bit of 3/32 heat shrink tubing or electrical tape. Be very careful, as this wire is only 26 gauge, so it’s very thin and easy to break.
Next, solder another six-inch length of wire to the bulb socket.
Run these two wires back to a toggle switch in the locomotive cab. I happened to have an 8-amp, 250-volt toggle switch that fits perfectly in a Marx locomotive cab. It’s total overkill–Marx trains pull about 2 amps at 14 volts–but did I mention it fit perfectly? It’s old, too, so it looks like it’s been there a long time. You can use any switch that fits, or that you can make fit. A thin piece of wood, such as a jumbo craft stick or paint stirring stick, cut to the width of the cab, would make a fine mounting bracket for a $2 Ebay switch or a switch you salvage from something else.
Just make sure whatever switch you use is a toggle switch, and not a momentary switch.
Now, when you cut the power and flip the switch into the off position, the locomotive will stay locked in the position it was last running in. You may have to try it out a couple of times to determine which position locks the reverse unit out. You’ll want to label the two positions once you figure them out, of course.
Locking out a reverse unit is an alternative to repairing it, but I’ve covered repair as well in a separate post.