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These are most likely a wireless passive IR hot body detectors, such as this one:

They actually detect changes in heat.

The receiver would be a home security console on the train attached to a bomb.

Although I don't know why one would go to so much trouble, unless you wanted to be absolutely certain that the train blew up at a particular place. Otherwise, why not just use a timer or something.

Although I don't know why one would go to so much trouble, unless you wanted to be absolutely certain that the train blew up at a particular place.

A commuter train would be full in certain places on its normal while empty in others. Also, an external timer would likely miss the train because a difference of mere seconds would cause a miss. With an external device as a trigger that is in a location where a train is normally full, all a terrorist has to do is board a train with a bag of explosives, leave it and walk away.

I use a couple of these things around my house so I can tell if someone comes in the driveway. The ones I use have a range of 200-300 feet, are waterproof, and the battery lasts well over a year. By themselves, they do nothing but transmit a radio signal when motion is detected.

All a terrorist would need is to have the receiver in a bag or suitcase wired to trigger some type of explosive and boom! They could carry the bag onto the train 500 miles from the trigger point, and ride safely and get off at some point in between, leaving the bag on the train.

This could be done at a time of their choosing, today, tomorrow, or 6 months from now. Once the trigger (sensor) is in place, all they need to do is to get the bag on the train. When the train goes by, it triggers the sensor and if the bag with the receiver is within range, that's all she wrote.

What worries me is that there are very high quality, reliable electronics (much better than X-10) available for $200 or less, and sensors like these could be put in place months in advance.