Track Detection

I am not a 2 rail guy so I have no way to test my proposed method. If operation is in one direction you could use 2 relays activated by reed switches mounted on each end of a block (spaced apart longer than the longest train run) between rails where you want an event to happen,such as activating a crossing signal. Magnets mounted on the underside of the locos or cars could work the reed switches. The first reed switch would latch the first relay coil through a set of its own NO contacts in series through the NC contacts of a second relay. As the train crossed the first reed switch, the first relay would latch and as the train crossed the second one the first relay would unlatch. A second set of contacts on the first relay could be used to switch the event. With this method activation could be made train specific by offsetting magnet or reed switch positions. This could also be done maybe using a short gap insulated rail without the reed switch. This method is much like the 3 rail method only using 2 relays and a latching circuit.

A single reed switch and relay could also be used with a timer circuit making a second relay unlatch it. Depends on what you are trying to do.

DPDT relays 12 VDC 24VDC,or 24 VAC could be used powered of a separate source. Most any toy train transformer. These relays can be had for $8 each with sockets. So you are looking at $16 per block not including reed switches and magnets which are cheap. Cheaper and more reliable and more flexible than IR circuits IMO.

Power sensing circuits require lighting or power usage in the first and last car. If this is done it is fairly reliable and provides good block detection. Dallee makes an excellent board or used to. The non common rail wire to the block is run through a sensing coil on the board. When power is pulled through, a SPDT relay is made which can switch an event. The relay on the board can be used to power other timing circuits or multi contact relays and staging multiple timed events is possible.

Dale H

Two rail detection is quite simple. I have used Integrated Signal Systems electronic components for years. The explanation to your questions are quite lengthy and complex however.
If further interested I would be happy to have a discussion off line where we could handle the subject in bite-sized pieces. My contact info is in the profile and I live in your area.
Regards,
Ben Brown