Model Train Technology

Merging Tracks Signaling C

Version C drops the signal controllers. Each relay is a DPDT: pole A is the kill switch for the other track’s detector, and pole B switches the signal for the track that has claimed the shared track between red and green.

Merging tracks relay interlock with a detector past the merge Relay K1 is energized by track 1: its pole A kills detector 2 and its pole B switches signal 1. Relay K2 is energized by track 2: its pole A kills detector 1 and its pole B switches signal 2. Detector 3, on the shared track past the merge, feeds both detector lines through a diode at each connection. Track 2 Track 1 Shared track Detector 2 Detector 1 Detector 3 Diode Diode B A Relay K1 (DPDT) Relay K2 (DPDT) coil coil to signal 1 to signal 2 1 kΩ Signal 2 1 kΩ Signal 1

Active detector signal Inactive wire Train

Why relay K1 switches signal 1

K1’s coil is energized only while track 1 holds the shared track, so its second pole is the natural switch for signal 1: released, it feeds the red lamp; energized, it feeds green. K2 does the same for signal 2. No signal controllers are needed.

Only the two lamp wires from each relay cross the tracks to reach their signal. Each signal’s common lead goes to the supply through a 1 kΩ, ¼-watt resistor. With a 12 V supply and LEDs around 3 V, that sets about 9 mA, and one resistor covers both lamps because the relay lights only one at a time.

Detector 3 feeds both detector lines through diodes, ahead of the relays, so it’s locked out by the same relays as detectors 1 and 2. A train must reach detector 3 before it leaves detector 1 or 2.