A suburban train usually will not lock up, unless there is damp rail, or worse, leaves on the tracks. The Metra suburban trains' locomotives have a "blended brake" feature that automatically bails off the engine brakes in favor of the dynamics.CSSHEGEWISCH
Several years ago, my suburban train went into emergency braking about a half-mile or so prior to my stop. I was already in the vestibule and recalled hearing the "WHAM" of the trainline but no wheels locking.
Back in the old days when cast iron brake shoes were used on passenger trains (at least they were on the CNW trains), locking the wheels was a distinct possibility; of course, the trade off was extremely aggressive braking on dry rail.
On freight trains, the main danger of emergency applications is there is bad weight distribution in the train (heavy loads behind empties). The empty cars will stop faster than the loads, and if the train blocking is wrong, the loads will "run in" towards the empties, the result being a possible derailment. If the engineer does not bail off the locomotive brakes, the effect will only be worse. (This scenario does not apply if the slack on the train is completely bunched)
Regarding the rail: I'll confirm what tree68 said.
