When the lamp atop this utility box is lit, so should be the grade crossing's
warning signals. If the signals malfunction, this lamp confirms the circuitry
is working at least that far. Or, if a train operator sees the lamp unlit,
he can report the grade crossing needs repair.
The lamp is a throwback to the early days of electricity when meters to check
current and voltage were not so commonly available. When automated grade
crossings were introduced during the early 20th century, they were one of the
more complex applications of circuitry. Elevators were, and still are, another.
I knew an old-time electrician who would test voltage by briefly touching bare
live wires: he'd twitch then report, "Yep, 220," for example.