This is all about how to sense when a train shows up in a block.
First, DCC is pretty much a square wave somewhere between 10 and 20 kHz, so while a steady DC voltage and a steady DC current can not be sensed through a transformer, current use on a DCC rail can!
So a simple circuit: coil, rectifying diodes and a small transistor, can easily tell you that there is something "active" (using current) on the track. Not only locomotives can be sensed, all your lighted cars, cabeese and wheel sets with resistors are sensed too!
The you add an LED and now you can tell if there is something without even using some special input/output board. So to make life simple, the LED could be on when there is nothing on the track (and now you know you have 5V to the sensor too...) and when the transistor turns on, it shorts the LED out and the 2-wire sensor will pull the 5V down to nothing so any digital input pin of a board, micro-controller, or Opamp can see it. Simply pull the sense line high with a resistor to 5V and the LED will be powered too.
(picture of sensor coming soon)
The TxCAN Sensor board:
12 (1st generation) (2nd gen never built), 28 (3rd gen), or 27 (4th gen) sensors can be plugged into the sensor board. The ground (GND) or common pin from the sensor goes to the pin closest to the edge of the board (see brown pins), and the sensor itself to the pin on the inside (see gold pins). 3rd generation sensor board shown below: (0C (circle) does not function on the 4th generation boards, and 1st generation boards only have 12 sensors, between 01 through 0C, as shown with pink and blue).
3rd generation sensor board.
The light brown sensors, 0D through 1C, will generate an interrupt on the SPI port-expander when one of these sensor's status change, and you will see a brief flash of the RED INT LED at the bottom right.
Create these sensors as MS+NNE01 through MS+NNE28 (for 3rd gen) in JMRI, where NN equals the decimal node number of the board; for example if the NN is 4008, the 3rd sensor on that board is MS+16392E03 and the last sensor, 1C, is MS+16392E28 (put a zero in front of the first 9 sensors (01 through 09) to help with the sorting in the list, you don't want them ordered as xxE1, xxE10, xxE11, xxE2, xxE20, xxE21, xxE3, etc).
Also note that JMRI will add the "MS" for you, do not type that part in, only the "+16392E28".