A 3-Railer’s Guide to Going 2-Rail

Converting from 3 rail to 2 rail O scale

You’ve decided to take the plunge and work with two rails instead of three, now what?


The first thing you need to realize is that O scale 2-rail, like most of the rest of the entire model railroad hobby, is driven by Standards. The National Model Railroad Association (NMRA), founded in 1935, establishes both Standards (documents designated with an “S”) and Recommended Practices (designated with an “RP”) to promote the interchange of products between different manufacturers. Visit their website and you will find Standards and RPs that cover every aspect of model railroading.

Of most interest to us in 2-rail O scale are the track and wheel standards. For example, you may see a reference to RP-25 wheels. That refers to NMRA Recommended Practice document #25 that defines the contours for model railroad wheels. The old O scale standard wheel was .175″ wide. The new standard, adopted in July 2009 with the help of OST Magazine, is 0.145″. Another Standard that is observed in O scale is the Right Hand Rule, as stated in Standard S-9, Electrical: “Positive potential applied to the right hand rail shall produce forward motion.” The right hand rail is the one to your right if you were standing at the back of the locomotive looking forward. The Right Hand Rule has implications later.

In general, 3-rail manufacturers pay no attention to NMRA standards so it would not be surprising if you found the above paragraphs surprising. In O scale 2-rail, standards matter.

Part 1: Track & Turnouts

Two-rail track is wired differently than 3-rail track. The fact that you’ve been running 3-rail trains with AC power is irrelevant because all modern 3-rail trains have DC motors inside them and circuitry to convert the AC to DC inside your locomotive.

So, what’s different? Let’s look at 3-rail track versus 2-rail track in the diagram below.

In the 3-rail track on the left, the two outer rails are connected to each other, the center rail (i.e., the 3rd rail) is insulated from the other two and conducts the power. The outer rails are the “common” or power return rails. In the two rail diagram on the right, each rail is insulated from the other. The upper rail (black) is the return rail and the lower rail (red) is the power rail. It needs to be mentioned that 3-rail trains do not have insulated wheels and axles because the wheels all touch the return rails.Only the power pickup is insulated. For 2-rail trains, all the wheels and axles are insulated on one side. For a diesel locomotive that means one power truck must be insulated on the right side and the other on the left side. For a steam locomotive, the steam engine is usually insulated on the left side and the tender on the right.

Remember the “Right Hand Rule”, if the power delivery rail is positive the locomotive should move forward, from left to right in our example. (It does not matter that most 3-rail trains run on AC and 2-rail on DC. The assumption still works for both.)

The track doesn’t look that different, that is until you look at a switch. Now, technically speaking, the thing that moves a train from one track to another is called a Turnout. A turnout has two assemblies: a switch and a frog. The switch assembly is the part that moves. You are probably used to calling a turnout a switch, so we’ll call it a switch for now. Let’s compare a 3-rail switch to a 2-rail switch.

The top image is a 3-rail switch. The outside rails are all connected and the 3rd rail is the power delivery rail. That funny looking thing in the center is called the Frog. The 3rd rail requires some tricky mechanical and electrical isolation so as not to cause a short circuit when a power pickup rolls over it.

The bottom image is a 2-rail switch. It is less mechanically complicated but the frog (in green) needs to be isolated electrically or else there will be a short circuit where the rails cross.

The genius of Joshua Lionel Cowen introducing 3-rail track and turnouts becomes apparent when we look at a reverse loop.

In the upper loop, the red and black rails never cross so you never get a short circuit. What Lionel did was trade off a little bit of complexity at the factory where they made the switch for the simplicity of wiring a model railroad at home. The factory does the hard work rather than you.

In the lower loop, it is immediately obvious that the red and black rails are going to cause a short circuit unless we take an active role in preventing it. We do that with gaps. Also note that the frog (green) needs to be treated separately.

For a 2-rail model railroad you must pay attention to polarity for reverse loops. Let’s look at a train going around the loop.

The train enters the loop at A and since the power rail is positive it moves from left to right around the loop. But, when it gets to B, it has to stop or it will cause a short circuit if it crosses the gap. Several things have to happen all at once for the train to keep moving.  You have to throw the points on the switch to let the train exit the loop and you have to reverse the polarity on the rails so that the train will now travel right to left. Like this:

The old way for DC powered layouts was to use a Double Pole Double Throw (DPDT) electrical switch and throw both the switch and the DPDT manually while the train is inside the loop. Now the train can exit the loop and keep moving in the right to left direction. You need to be concerned about these kinds of polarity issues for reverse loops, wyes and turntables.

It looks complicated and it is if you are wiring for old time straight DC, but modern electronics comes to the rescue. You can buy automatic switching circuits that will do all the thinking for you and let you just run trains. There are electromechanical systems (e.g. Azatrax ) that use relays and sensors if you need to be concerned about polarity differences, for example, using straight DC or a polarity sensitive command system like DCS with Proto-Sound 1 or 2. And there are completely electronic systems like the Dual Frog Juicer for DCC, TMCC and DCS with Proto-Sound 3. (I’ll discuss control systems next, be patient.)

Let’s go back to the 2-rail turnout. There are a couple of other things that we need to discuss. On some older turnouts the two point rails might be soldered to a throw bar that conducts electricity and the point rails might also be connected to the frog. That means that points and frog are one electrical entity (green lines) and the polarity of that unit depends on which stock rail (red or black) it is touching.

Relying on the contact between the point rails and stock rails for electrical conduction is not wise. It also means that there are more chances for short circuits to occur between the point rails and the stock rails.

It is much better to have each point rail be electrically connected to its adjacent stock rail all the time and insulate the points from each other and from the frog. Like so:

A turnout set up like this is often called “DCC Friendly”. As it turn out it’s also DCS and TMCC friendly and works just fine with straight DC too. This is how a commercial turnout from Atlas is set up right out of the box. But that still leaves the frog, the green bits.

There are folks who prefer to leave the frog dead, i.e., unpowered. That’s okay if all your engines have a powered wheelbase longer than the frog. I happened to have one 4-6-0 where only the drivers pick up power, not the lead truck, and I have one custom turnout with a frog that is longer than the wheelbase of the 4-6-0. The result is that when the pick-up side drivers are sitting on the frog, if the frog is dead, then so is the locomotive. The solution is to power the frog with either a mechanical switch, a relay or an electronic circuit. If you do that, you will always have the turnout powered properly.

Here I’ve shown a switch machine that has an electrical contact that moves when the points move. This way the frog is always powered in the proper direction for travel through the turnout. Atlas turnouts have a copper tab the sticks out one side of the turnout and that tab is connected to the frog.

Most commercial switch machines have contacts to power the frog. The Tortoise has them. I use the Tam Valley Depot Singlet Servo Decoders with RC servomotors to throw my switches. The decoder board has a connector for a DPDT relay (also sold by Tam Valley) that I use to power the frog. When the servo moves through it’s arc to move the points, the board sends a signal to change the relay contacts. Atlas makes a Snap Relay that is used to power the frog in conjunction with their switch machine. Tam Valley also makes the Frog Juicer, a completely solid-state frog polarity reverser.

With modern electronics, it is no more difficult to wire a 2-rail layout than a 3-rail layout. With DCS you have issues with signal strength and you need to use star-wiring. With Lionel Legacy and TMCC you have ground plane issues and you need to be sure there is a ground plane next to or under all the rails. With 2-rail layouts you have to watch the polarity and make sure you do not have the possibility to create a short circuit.

Both Kalmbach and Carstens sell books about basic wiring for model railroads. Get you one! Read it and understand it before you do anything about building a 2-rail model railroad.

And finally a few words about 2-rail track materials and sizes. In the “old days”, 2-rail track came in two basic materials: steel and brass. Today, almost all 2-rail track is made from Nickel-silver. Steel rail is still considered the best for maximum traction with steel wheels. The problems with steel rail are: it is not easily soldered (you need acid flux); it rusts; it’s hard to find; and it is taller than most other rail. Brass is a copper alloy that conducts electricity very well and is easily soldered. However, brass oxidizes easily and that oxidation kills the electrical contact to wheels. Brass also does not look like steel rail, nor does it weather like steel. The answer to the problems of brass and steel was nickel-silver. Nickel-silver is another copper alloy with a bit of nickel added. It is silver in color and looks more like steel. It also does not oxidize as easily as brass and solders just as well. It’s one downside is that it is slightly less conductive than brass but the other pluses make up for this.

O Scale rail comes in a variety of sizes called Codes. The most popular size today is Code 148. This means the rail height from the base of the web to the top of the head is 0.148 inches. Code 125 rail is 0.125 inches, and so on. Different Code sizes represent different rail sizes in the real world. Code 148 would be typical of a modern Class 1 railroad or a steam era railroad like the PRR. Code 125 rail might be a modern branchline or a steam era mainline. Code 100 might be used in yards. Code 83 would be used for narrow gauge and some older branches. If you want to learn more about prototypical track and turnouts check out Mike Cougill’s book “Detailing Track” at the OST Publications website.

Sources for Track & Turnouts

  • Atlas O: Code 148 track and turnouts
  • MicroEngineering; Code 148 flex track in O standard gauge and Codes 70, 83 and 100 for On30.
  • San Juan Car Co.: Code 100 #6 On3 turnouts
  • BK Enterprises: Code 148, 138, 125 and 100 custom-made turnouts for O standard, P48, On3 and On30.
  • Protocraft: Code 125 P48 flex track
  • O Scale Turnouts: Coming soon, Code 148 custom turnouts in O standard gauge.
  • MicroMark: Code 148, 125 and 100 O standard flex track

Next: Control Systems & Power Distribution