There is some talk running around the net about the age and condition of the subway cars that crashed.
Doubtful. Railcars last forever, especially stainless steel ones. The air brakes were invented in 1880 and work just fine in 2009. One train rear ended another, which means the following train ran a block signal AND failed to see the other train in time. Or the block signals broke. All rail systems have block signals. The signal at the entrance to an occupied block shows red, the signal for the block behind the occupied block shows yellow, and further back block signals show green.
So, we have maybe three possibilities. The operator of the following train failed to obey the block signals (sudden heart attack? texting while training? who knows, she died in the crash). Or the brakes failed. Or the block signals failed.
By the way, the Knoxville TV station quoted above mentions "roll back". That's a new one. Trains have friction brakes just like cars. Put the brakes on and the rail car comes to a stop, forward backwards it's all the same to the brakes.