I read about a system that was proposed for the US, but everybody is balking about the costs. The real problem is that in order to work ALL of the trains using the tracks must be equipped. It may have been in a professional publication such as Intech.
In one aspect of the system, the system is to know the position of every train no matter where it is. A tunnel, where cell service is not available etc. So, it does not rely on a single technology.
One such system
http://en.wikipedia.org/wiki/EBICAB
http://en.wikipedia.org/wiki/Automatic_train_stop
**broken link removed**
These are not the product that I read about. Most of them are local. The US and others needs a global product were everyone is using the same technology. Trains need the equlivelent of an Air traffic Controller as well for each region.
It might use the cell towers, or land transmission or RF to a very local receiver/transmitter.
You do have to let the system control the trains and it as to be as foolproof as possible.
You might employ track obstacle detection at the crossings.
The tracks generally need to be elevated over roads if at all possible.
Signaling is also integrated into the system.
Unfortunately, it's difficult to prevent someone from following the tracks for a shortcut, but collisions based on conditions and stupid stuff all require for one to know where the trains are at all times. A transponder for trains rather than aircraft.
There is te added complexity of transmission and detection and signals and crossings.