Track / Train detection.

I am taking a transportation security class and decided to do my term paper on the railroad. Even though I have been a model railroader, I don't really know much about how a railroad truly operates. I figure this is a good opportunity to learn.
If there is a break in the track, is there a system in place to identify this so it can be fixed before a train finds it? Such as a continuity signal.
Also, I imagine that train locations are tracked by GPS now, at least the locos if not the cars themselves. Is there another method, or what was the old way of doing it?
Is there a sensor on the tracks which trips when a train passes by to activate the signal to show that a train is in a block?

Thanks

In the United States train detection in signaled territory is carried out by track circuits that can detect a break in the rail and usually floods, derailments and metal objects such as motor vehicles on the tracks. It can also be easily "shunted" by MoW crews to indicate the track is obstructed. In Europe there is a tendency to use axle counters due to the very late adoption of train detection technologies. These do what they say on the tin and count axles in and out of a block to infer that it is not occupied. This system works best with centralized signaling schemes and where trains don't stop, reverse or set off cars in the middle of the block.

No train locations are tracked by GPS as of yet except for some pilot schemes in otherwise unsignaled territory. Normally those train crews simply report clear of fixed blocks or locations. GPS tracking might automate this process, but GPS is highly vulnerable to spoofing and anyone with about $500 worth of equipment can make a train think its on the moon.

Some rail equipment have RF tags on the side and sensors in the field to identify where it is, known as AEI(Automatic Equipment Identification). But this is not used in controlling traffic or signals, just to keep tracks of what is going where.

Amtrak has an internal web-site which they can use to view the exact location, and speed of any of the electric motors and Acela trainsets.

Even all Amtrak diesels have tracking and Mechanical info constatly transmitted to operations dept.