A simple effective way to demonstrate railroad freight transportation at public exhibits and museum displays.
 
This is a single-track oval with a passing siding hidden behind the scenic backdrop. The blue train consists of hopper cars loaded with coal or ore. It travels counter-clockwise around the track, appearing to take its load from the mine to the steel mill.
 
After one trip around the loop, the blue train stops on its siding. The red train, consisting of similar but empty hopper cars, then makes a clockwise circuit around the loop. It is taking the empty hoppers back to the mine. When the red train disapperas behind the backdrop and returns to its siding, it stops, the blue train starts again, and the cycle repeats.
 
The theme can be developed further by adding a similar track to the right of the first one:

How it works: The MRD2-Exhibit circuit module has two infrared train detectors. The sensors for one detector are placed at the end of one of the passing sidings, and the sensors for the second detector are placed at the opposite end of the other passing siding.
 
The track power is DC (not DCC). Polarity determines the direction of train movement. In the diagram below, when Relay1 is 'on' and Relay2 is 'off,' the inside rail is positive and trains will travel clockwise. If Relay1 is 'off' and Relay2 is 'on,' the outside rail is positive and trains will run counter-clockwise. If Relays 1 & 2 are both 'off,' all trains stop.
 
Diodes allow trains in the sidings to only move in one direction.
 
For AC and DCC powered trains, see the Exhibit Controller instruction sheet [pdf].
 
Set the MRD2 Mode switch to TO/M. When the train 1 has entered its siding and trips its detector, the controller turns off Relay2, waits 5 seconds, then turns on Relay1. This stops train 1 and starts train 2. When train 2 returns to its siding and trips its detector, the controller turns off Relay1, waits 5 seconds, then turns on Relay2. This stops train 2 and starts train 1 again.

Spring switches - a spring lightly holds the points to one side, so a train entering the switch always takes that route. But the spring is light enough that a train exiting the other siding can push the points aside and not derail. Slow motion switch motors - continuously powered by a DC supply. The switch position is changed by reversing the supplied voltage.   Wire both switch motors in parallel to pins 1 & 3 of the white connector on the loop controller, shown at right. Switch machines must draw 10 milliamps or less. SwitchMaster™ and Tortoise™ machines meet this requirement. SwitchMaster™ users may replace the stock 1200 ohm series resistor with a 470 ohm resistor to preserve the 4-second operating time.   An alternative way to control slow motion switch motors is with a third DPDT relay, as shown in the wiring diagram above. Twin-coil snap switch machines - Relay3 not required, but does require an MRD2 turnout controller in addition to the MRD2 loop controller.

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