The first method uses one infrared train detector to control power to an electrically isolated section of track. It works for trains with a single engine. The isolated section should be slightly longer than the longest locomotive used on the layout, plus stopping distance. For 3-rail layouts, isolate the center rail.

In this diagram, the red and blue trains travel clockwise around the loop. The blue train is stopped, its locomotive is on the isolated section of track. The red train is moving, and is approaching the train detector.
 
When the red train activates the detector, the detector turns on a relay, which in turn switches power to the isolated section.
 
As long as the detector is sensing the red train, the isolated track section will have power, so the blue train can move far enough to exit the isolated section.
 
The two trains continue moving clockwise. When the red train reaches the isolated track section, it will stop until the blue train reaches the detector.
 
To control three trains, use two isolated sections and two detectors.
 
If trains have multiple unit locomotives, then a second detector is required. This is because in the diagram above, the first locomotive will enter the dead track section, but other engines behind it will still be on powered track.

For trains with multiple engines, the isolated section must be longer than the longest train, plus stopping distance. A second detector (D2) at the entrance to the isolated section keeps power on until the train is completely in the section. Power will be restored when the other train activates detector D1.
 
A third detector, D3, is needed if trains have a pusher helper or distributed power units (a powered locomotive at the rear of the train). Detector D3 keeps power to the isolated section on while the train is leaving the isolated section. It prevents the possiblility of the rear locomotive being caught in the isolated section if detector D1 turns off early.
 
The outputs of detectors D1, D2 & D3 are wired in parallel, so the power relay is on when any of the detectors are sensing a train.
 
See the detector installation examples page for tips for installing infrared train detectors.
 
Want to automatically run two trains in opposite directions on a single track loop? Click here for the next page.