Wiring Marx accessories isn’t too hard, and Marx accessories are forgiving. You can just run two wires to them, somehow, and they’ll probably work. But that’s probably not what you’re after. You probably want efficiency in terms of how much wire you use, and you probably want to hide your wires. Marx’s design allows for some clever tricks hobbyists have devised over the years.
The main rule to remember is to use around 20 AWG wire and connect the non-insulated binding post on the accessory to the common post on your transformer, and the insulated binding post to the hot post, using the dedicated accessory terminals if your transformer has them. This allows you to do some tricks when it comes to saving wire.
AWG of wire to use for accessories
First things first: While the trains themselves benefit from heavy gauge wire (14 or even 12 gauge wire isn’t out of line), you don’t need a lot for lights and accessories.
For accessories, 20 AWG wire is overkill. It won’t hurt anything, but it’s more than you need. On an 8-foot run using even super-thin 24 gauge wire, you’re looking at a voltage drop of around 2 percent. Old-school RJ11 telephone wire is usually 22 gauge, and computer network cables typically use 24 gauge wire. So you can salvage wire from old phone or network wiring for your accessories.
I’m more than comfortable wiring my accessories with 22-gauge wire. Today it’s easier to salvage 24-gauge wire, in the form of old computer network cables. Computer network cables do go bad, so your employer’s IT department probably throws away quite a bit of it. If you ask nicely, your IT department will probably save bad cables for you.
If you use 24 AWG wire, simply double up on it.
Wire connectors for thin wire can be a problem. The minimum for blue wire nut connectors is two 22-gauge wires. So be sure to pick up a supply of blue wire nuts.
Which terminals go where
Marx transformers from cheaper train sets only have one set of posts, which connect directly to your track. You can connect accessories to these posts too, or power them from the track, but you’ll be happier running them from a dedicated, steady power source. Costlier Marx transformers had a set of posts dedicated to accessories.
You can use multiple transformers if you wish, as long as you phase them so they alternate in sync. Here’s how to phase two train transformers.
You can also use a PC power supply to power your accessories. An inexpensive PC power supply can power a small city’s worth of lights. Even a $20 power supply will supply twice as much power as Marx’s biggest, most expensive transformer, for about 25 percent of the price. I used a 75 watt transformer for lights on my layout and by the time I exceeded 20 light bulbs on my layout, that transformer couldn’t handle it. A typical PC power supply can easily handle more than 40 bulbs. Another popular and cost-effective option, if you have accessories that need AC power, is to repurpose an outdoor landscape lighting transformer.
Finding your common post
If you’re using a Marx transformer, you’ll need to know which of your posts is the common post and which one is the hot post. Marx didn’t label them, unfortunately. The easiest way to find out is to plug the transformer, don’t turn on the throttle, and then check the posts with a voltmeter. Connect the voltmeter to one of the accessory posts and touch the other lead to each of the track posts. If you can get a 13-volt reading touching to either of the track posts, that’s your hot post. If you can’t get a 13-volt reading from either track post, that’s your common.
In my experience it seems Marx usually put the common posts on top, but that’s merely my observation.
Flip your accessory over. You’ll see that one of the binding posts has a wire connected to it, while the other binding post just connects straight to the accessory. The post with the wire is your hot post. The other binding post is common. If you reverse this, the accessory still works, but if a running train bumps it for some reason, it will cause a short.
Now here’s the trick. You can run the hot wire from your transformer’s hot post to the nearest accessory. Then jump from that accessory to the next accessory’s hot post, and so on, until you’ve supplied power to every accessory. But you can take shortcuts with the common wire. You don’t have to run that one all the way back to the transformer if you don’t want. You can connect that post to any nearby outer rail if you want. As long as you connect just one of your accessories to the track, you can just connect all the other accessories together, and they’ll light.
This cuts way down on how many connections you have to make in the limited space on the transformer’s posts, and how many wires you have zigzagging under your table.
Wiring accessories with control buttons
To wire accessories with control buttons, I recommend running the hot wire straight to the transformer’s hot post. Connect the common post to one wire from the control button. If you want, you can connect the other wire from the control button to the transformer, since it’s probably going to be nearby. But you can connect the other end of the wire to any nearby outer track rail, or the common post on any other nearby accessory if you wish.
Using magnets and contacts
The biggest trick I want to cover is using magnets attach Marx accessories and contacts to power them. I’m not sure who invented this method but I’ve seen several people do it over the years.
Marx provided terminals on the top of the accessories to attach wires. But you can supply power from below just as easily. All you have to do is make electrical contact with the screw heads. I’ve done this before by attaching wires underneath. But why use wires when you can just use electrical contacts and hold the accessory in place with a magnet?
You can use magnets for the contacts themselves, but it’s cheaper to use one magnet to hold the accessory in place, and a couple of strips of brass or copper or even aluminum tape for contacts.
Making a template
The first step is to make a paper template for the accessories you have. Place a piece of paper on the underside of your accessory and trace out the position of the screws and the edge of the base. Cut out a piece of thick cardstock to fit within the base. Chipboard or binders board works well.
A square of 2.5 inches fits nicely under square-base accessories. Cut two thin strips of brass or aluminum and position them to meet up where the screws meet, about a quarter-inch from two of the corners..
Bend the contact into a V shape. If you don’t mind soldering, solder feeder wires to each contact. If you don’t want to solder, drill a hole in one leg of the v to accept a machine screw. Attach the contacts to your wooden insert, positioning them according to the template you made. Glue a magnet to your insert in a position away from the screws. Shim it up with washers so it reaches the accessory if needed.
Drill holes in your table for the feeder wires or for the machine screws. Drop the screws or the wires down through the table. Attach the insert to your table with a wood screw. Now you have either two machine screws or two wires sticking down from your table to power your accessory. None of the wiring is visible from the top of the layout. Better still, the magnet and the insert hold the accessory firmly in place until you move it.
Round-base accessories are tougher because their screws don’t protrude down as far. For those, you can build your contacts up taller, or you can replace the screws with longer 8-32 machine screws, along with an extra nut. One at a time, remove the old Marx screw. Thread a nut onto the replacement screw, cinching it down close to the head. Thread the replacement screw up into the accessory, attaching the wire and the insulated washers the way they were, then attaching the nut and the binding nut to the top. Position the screw so that protrudes down about 1/8 inch from the accessory base, then cinch the nut you added up to the top of the accessory. Now you can power your round-base accessory from contacts placed on your layout.