Below the white X-shaped railway crossbuck at a level crossing, you notice a small tab showing the numeral 2. What is it telling you, and why should it change how you cross?

Correct answer: A. There are two sets of tracks — a train on one can hide a second train on the other

There are two sets of tracks at that crossing. Book 5 describes the supplementary NUMBER OF TRACKS tab as displaying "a numeral corresponding to the number of tracks at the railway crossing location," and says the crossbuck "must be appended with the NUMBER OF TRACKS tab sign, when there are two or more tracks at the crossing." The reason it matters is the one that catches people out: a train on the near track can completely hide a second train coming the other way on the far track. Seeing one train clear the crossing is not the same as the crossing being clear. Wait until you can see right across every track, and never start over a multi-track crossing without room to get all the way across.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 5 — Regulatory Signs (December 2021), NUMBER OF TRACKS Tab Sign (RA-6S TAC)