Three-rail toy train track provides a distinct advantage for the toy train operator wanting to use the position of trains to operate trackside accessories or to control train action: the track itself can be used for a low-cost, practically invisible, mechanism for detecting a train's location.

The wheels and axles of engines and rolling stock can be used to close an electrical connection across track outside rails.

By insulating one outside track rail from the other, a train's wheels and axles will provide a useful electrical action that can be used for operating accessories when trains enter a designated track area.

O22 switches employ this same mechanism to provide a non-derailing function.

Lionel refers to this action as "wheel-operated electrical switches" and explains it in OSS and OCS Track . Lionel introduced its first "wheel-operated" product in 1921. Our History web page gives more details.

High rail track sections with an isolated outside rail are commercially available, but only the conventional length straight sections. Any track section, straight, long straight, or curves, can have an outside rail isolated using the technique covered in this paper: Insulating Outside Rails of High-Rail Track .

An isolated-rail track block is used to operate an accessory by electrically connecting the accessory to the insulated rail.

Additional electrical connections are made from the accessory and the accessory power transformer post. The other accessory power transformer post is connected to train power "common."

If the same transformer is used for train power and accessory power, the "common" connection is supplied by the transformer, as in the following diagram.

The "A" handle controls the train; the "B" handle is set to power the trackside accessory.

The accessory operates whenever train wheels are across the outside rails of the isolated-rail track block. This action is independent of train power; the accessory will operate even with train power off.

An isolated-rail track block provides a convenient technique for operating trackside accessories. It does, however, have some problems:

• The technique can only be used to turn an accessory on when a train enters the isolated track, off when it leaves the isolated track.
  Accessories such as block signals and flashing crossbucks can't be operated properly from a directly connected isolated track block.
• Because accessory power common and train power common must be connected together, not all available transformer voltages can be used.
  The KW transformer shown previously illustrates this problem. Since the "U" terminal must be connected to the outside rail for train power, only the "A," B," 6V, or 20V terminals can be used for accessory power. The desirable 14V terminals cannot be used, since neither terminal can be connected to the outside rail.
• Operating multiple accessories from one track block or a single accessory from multiple track blocks can present some tricky wiring.
• The accessory is activated whenever a train is in the connected track block. If a train is stopped in the track block, the accessory is activated continuously. This can be a problem for solenoid operated accessories that can overheat if activated continuously.
• All the power to operate an accessory is being carried by train wheels and axles. Not a problem for low current lights, but some solenoid activated accessories can draw two amperes of electrical current.
• A derailment can put track power through the accessory.

Three-Rail Innovations' controllers utilize isolated-rail track blocks for a low-cost, invisible mechanism for detecting a train's location, but without any of these problems.

Three-Rail Innovations' controllers are the only practical way to properly operate AC trackside accessories as toy trains move around a layout. Our products can control accessories, track switches, trains, or layout lighting.

MORE ABOUT The Two-Relay Controller

Three-Rail Innovations' Two-Relay Controller operates AC or DC powered trackside accessories, controls train power, operates track switches, and makes running your trains more interesting and safer.

Add lighting action to your train layout by changing boring, always on, lighting accessories and lights in structures to cycle on and off, follow prototypical day/night cycling, turn on and off as trains move into yard areas, or respond to operator action.

Three-Rail Innovations' Automatic Train XChange controller uses isolated outside-rail "stop" blocks to allow running two trains on a single track loop, exchanging trains with a passing siding, and many other layout configurations.

The E-Unit Cycler uses outside-rail "stop" blocks to implement a back-and-forth layout.

Three-Rail Innovations' Train Block Controller uses isolated outside-rail track blocks to implement a Block Control system for running more than one train on the same track.