Track circuits are not redundant because redundancy does not equal safety. Track circuits are designed to be fail-safe, if anything bad occurs the system should not permit the vehicle to move. Fail-safe systems are prone to failure and shifts in calibration which is why most manufacturers and the FRA require periodic maintenance and performance checks. Redundancy can make a system more reliable but it won’t improve safety unless the redundant systems fail-safe and dissimilar.

This accident appears to have been caused by a track circuit that failed to detect a train occupying a block. When this happens the approaching trains will be commanded at full speed as if there is no train ahead. A worse case failure scenario. How can this possibly happen?

Vehicles do not shunt the rails perfectly and some of the detection signal can leak past the axles and go to the receiver. If this leakage is too high the receiver will not properly detect a train, hence calibration and maintenance of track signals is very important.

http://en.wikipedia.org/wiki/Track_circuit