Why was "13" written on the wall as the train leaves District 12 in Catching Fire?

Question

Why was "13" written on the wall as the train leaves a tunnel from District 12 in Catching Fire?

This 13 would seem to me to label a tunnel to District 13 but they're on a train leaving this tunnel, so that makes no sense.

Answer 1

Maybe because it's a B and not 13?

There is another one which is to the right which you can make out through the fence.