You are stopped at a red light a short distance beyond a level crossing when the crossing bells start and, moments later, your light turns green out of its usual order. What has happened?

Correct answer: B. The crossing has preempted the signal to clear vehicles off the tracks before the train arrives — move off promptly

The signal has been preempted by the railway crossing so that it can get vehicles off the tracks. Book 12 explains the arrangement: where "a railway crossing is in proximity to an intersection such that vehicles queue toward the tracks, and inadvertent vehicular stoppage may occur on the level crossing, it is necessary to provide a preemption phase to clear the approach before the train arrival", with the timing calculated to clear the tracks plus a margin of safety. The same interconnection guards against the reverse problem, where lowering gates cause traffic to queue back into the intersection. It may also shut down turns toward the crossing, "by eliminating a phase, by activating arrow signal heads, by activating blank-out signs, or some combination". So an unexpected green near a crossing is not a fault — it is an instruction to move, promptly.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 12 — Traffic Signals (July 2024), Section 3 — Preemption and Priority, Preemption For Railway Crossings