The signalling system enforces the rules automatically - it's (supposed to be) designed in such a way to prevent unsafe situations from occuring. And back in the "good old" days, the rules that the system enforced weren't nearly as stringent.
And then Russell Hill happened.
Since then, they've modified the signal system to provide more positive - and structured - enforcement. Some features, such as grade time (where a signal has to count down a pre-set time before it clears), have been enhanced and improved. Others, such as station timing (where a signal will allow a train to bypass it even if red after a pre-set time has elapsed) have been removed. Manual call-ons have been largely eliminated.
For the record, one of the things that changed after Russell Hill that had nothing at all to do with the accident was the ending of high-rate operation on the B-D. That was because David Gunn looked at the mechanical records and found that the traction motors and gearboxes were failing at a considerably higher rate than when they were running in low-rate.
Dan
Toronto, Ont.