Cards that senses the presence of a train on the track by the train's current consumption.

 The card is connected to the track from one of the 8 outputs marked "A".
Both sides of the track feed should be connected to the input terminal marked "A" and "B".

The card works by detecting a voltage drop over a diode on the card. The voltage drop will be around 0.3 volt.

The card will use current from the track for its own need and will not need any additional current supply.

The card is isolated from the decoder side through opto-couplers.

8 track can be detected by one card.

The outputs on the card is connected to the inputs of the switch decoders or the signal decoders.

The sensitivity is high and around 0.3 mA can be sensed.

Outputs to the decoder is of the type "Open collector".
 

Example of connection to three tracks (click on the image to make it bigger):