Use infrared train detectors to avoid collisions at model railroad track crossings

A level track crossing, often called a 'diamond,' creates visual interest on your model railway. It enables you to fit more track in limited space. It also affords operating challenges.
 
The obvious risk at a diamond track crossing is that of train collisions. Train operators must be familiar with the layout and must remain vigilant not only of their own train's location and direction, but also of possible conflicting traffic on the intersecting track.
 
Installing an automated collision prevention system at your diamond crossing will reduce the anxiety when novice or guest engineers are operating on your layout.
 
The diagram below shows how to add automatic crash protection by using infrared train detectors and a relay.
 
This schematic shows a north-south double track mainline crossing an east-west double track mainline. The principle can be adapted to cover any number of tracks.
 
Here, detectors are placed on the north-south tracks, and insulated rail sections are created on the east-west tracks. When any of the detectors sense a train on the north-south tracks, power to the insulated sections of the east-west tracks is cut off. This prevents an east- or west-bound train from hitting a north- or south-bound train that is already at the crossing.

The detectors shown are Azatrax MRD1 infrared train detectors. They use their own infrared light, so they can be used in bright or dark environments. The sensing elements (IR LED and photo sensor) may be placed in the roadbed flush with the ties, or can be placed on either side of the tracks. See the detector installation instructions and the examples page for more information.
 
At the north end of this example, detectors 1 and 2 have their sensors in the roadbed. One detector is required for each track.
 
On the south side, the sensors for detector 3 are to the side, looking across both tracks, so only one detector is needed. You can use either method, whichever best suits your situation.
 
How it works: When any of the train detectors senses a train, it closes its output relay contact, Q1 and QC. This energizes the relay, which interrupts power to the insulated rail sections.

Protect the contacts on the detectors! The MRAPR Relay has built-in protection, so no additional components are needed. It can also be used with either AC or DC accessory power. Other relays will need a diode across the coil to protect the detector contacts from the inductive kickback.
 
The override switch is a momentary, normally-closed switch. It can be used to manually restore power to the track.
 
Sensor placement: Place the sensors so they will sense a train before it interferes with the intersecting track. The distance between the sensors should be less than the length of the shortest train using the track.
 
Insulated rail joiner placement: The distance from the insulator to the crossing should be the length of your longest locomotive (including multiple units) plus stopping distance.
 
To protect both sets of tracks: The system shown in the upper diagram only protects half of the crossing. Use the next diagram to add sensors on the east-west tracks and a second relay to control insulated sections of the north-south tracks. Now all your tracks will have automatic collision protection.
 
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