For a long time I thought this was an episode of "Fury" 1955-1959 but just read through a Episode Guide and didn't find it.
Two boys, one the series regular, are walking through a long railroad tunnel. The non-regular injures his foot and cannot walk without help. It is too late to turn back before the train enters and with the slow going, they might not have time to reach the other end.
Thanks ahead of time to the Boomer who remembers this one.