When a train layout has more than one transformer circuit connected to different parts of the same track, a train will pass from one transformer circuit to another where the different transformer circuits are adjacent.

An engine's center-rail power pickup rollers bridge the separate transformer circuits and cause a SHORT CIRCUIT.

The following diagrams illustrate a (simplified) engine moving from a track block which is powered by one transformer circuit (A, 12 volts) to a track block powered by another transformer circuit (B, 16 volts).

When the engine center-rail pickup rollers pass over the plastic pin and contact the center-rail of block B, with the trailing roller still in block A, a SHORT CIRCUIT occurs between transformer circuits A and B.

In the example, 4 volts (16 - 12) are shorted. This shorting is demonstrated by any of the following:

• Excessive sparking at engine pickup rollers and wheels.
  • There will be more sparks when the voltage difference between the adjacent blocks is greatest. Observe in a darkened room for best viewing.
  • If the track blocks are on separate transformers that are not "in-phase," sparking will be serious, based on the sum of the adjacent blocks' voltages. Welding can occur.
• Spark pits on the center rail near the break and wear on the pickup rollers.
• Spurious activation of engine E-Unit/direction control.
• Tripping of circuit breakers.
• Damage to interconnecting wires or circuit-board traces in engines with center-rail pickup rollers in each truck.
• Damage to interconnecting wires in passenger cars with center-rail pickup rollers in each truck.
• A train with lighted passenger cars continuing to "lurch" into an unpowered track block when it should stop.

The short circuit may not be obvious when the track blocks' voltage settings are nearly the same and an engine moves quickly over the powering boundary. Set the first track block for a slow speed (low voltage) and the second for a fast speed (high voltage) to clearly demonstrate the short circuit that occurs.

This short-circuit problem can be solved by "passing" center-rail power from the block ahead to the block the engine is in just before the center-rail pickup crosses the block powering boundary.

This way, when the center-rail pickup crosses the block boundary, both blocks are at the same voltage.

The following simplified sequence shows Three-Rail Innovations' Two Relay Controller "Passing" center-rail power from a following track block to avoid shorting the transformer circuits. Track block A is powered through a 2RC relay. The C (common) terminal is connected to block A center rail. Transformer circuit A is connected to the NC (normally closed) terminal. Transformer circuit B is connected to the NO (normally open) terminal. Only track center rail power connections are shown.

A train is moving from the track block powered with transformer circuit A to the block powered with transformer circuit B. The following diagram shows the engine center rail pickup in track block A.

The relay is OFF, so transformer circuit A is powering the block through the NC terminal.

As the train wheels reach track block B, the outside rail connection causes the 2RC to turn the relay ON. Center rail power for block A is switched to transformer circuit B, through the relay NO terminal. Track blocks A and B are both powered from the transformer B circuit.

As the engine pickup rollers reach track block B, NO short circuit occurs: both track blocks are powered from the same transformer circuit.

When the last wheels of the train clear track block A, the relay is turned OFF. Power, and control, for track block A is restored to transformer circuit A.

Note that a simple relay connection will not reliably perform the "power passing" function: wheel bounce and dirty wheels will cause relay chatter and a short circuit through the relay contacts . Power for track block A would be changed as long as a train is in track block B. Also, the 2RC adds direction control so reliable "power passing" can be performed in both train directions.

The following drawings show the 2RC connections for the "Power- Passing" feature for a train in either direction across a change in center-rail track power.

The track has two "blocks," each powered from separate (A, B) transformer circuits. A plastic insulating pin in the center-rail divides the two blocks. The blocks may be of any length.

Each block has a section of isolated outside-rail track adjacent to the power block divider. The isolated outside-rail blocks are established with insulating plastic pins for a length longer than the largest span of wheel trucks; they need not be the entire length of the powering track blocks.

The 2RC is jumpered for the NO V vertical jumper feature set and "Power Passing" for both Relay-1 and Relay-2. It is powered by 12 - 14 volts AC connected to power terminals. 8 - 20 volts AC is connected to Track Common and Input-1-B and Input-2-B terminals for powering controller inputs.

• Input-1-A is connected to the isolated outside rail of block A.
• Input-2-A is connected to the isolated outside rail of block B.
• Train transformer(s) have one post connected to track common. The center-rail power circuit for A is connected to the Relay-1 NC (normally closed) terminal and the Relay-2 NO (normally open) terminal.
• The center-rail power circuit for B is connected to the Relay-2 NC (normally closed) terminal and the Relay-1 NO (normally open) terminal.
• Center-rail power for track block A is connected to the Relay-1 C (Common) terminal.
• Center-rail power for track block B is connected to the Relay-2 C (Common) terminal.

With no trains in either track block both relays are OFF. Power for block A is supplied by transformer A through the Relay-1 C (Common) and NC (normally closed) terminals. Power for block B is supplied by transformer B through the Relay-2 C (Common) and NC (normally closed) terminals.

As a train enters block A and reaches the block A isolated outside rail, the 2RC recognizes the train, but the relays, and powering, are not changed.

The instant the train wheels reach the B isolated outside rail block, Relay-1 is turned ON and power for block A is switched to transformer circuit B, through the Relay-1 C (Common) and NO (normally open) terminals.

As the train proceeds, the center-rail pickup rollers reach the block B center rail, Relay-1 has been turned ON, so

NO TRANSFORMER SHORT CIRCUIT OCCURS!

Relay-1 remains ON until the train passes out of the block A isolated outside rail track block, then turns OFF. Then, another train entering track block A, but not the block A isolated outside rail section, will be powered, and controlled by transformer circuit A.

If the train passes out of block A, then is reversed and re-enters block A, Relay-1 is turned back ON and power, and control, for the train is still from transformer B.

Once the train has passed out of the block B isolated outside rail section, everything is reset and ready for another train.

Note that Relay-2 was not affected by the train moving from track block A to track block B.

If a train enters from the block B direction, a similar action occurs, with Relay-2 and power switched from transformer B to transformer A. Relay-1 is not affected.

If your train layout has separately powered track blocks on the same train route, you need Three-Rail Innovations' Two Relay Controller to prevent short circuits.

Click the bar for more information about Three-Rail Innovations' Two Relay Controller.