RAILROAD TERMS AND SLANG - All Aboard: The Complete North American Train Travel Guide - page 320

air signal: A separate air line runs the entire length of a passen-
ger train. When activated by a valve in each car, it beeps in the
locomotive cab. The conductor can use this means to signal the
engineer but seldom does, since normal communication is carried
out by handheld radios.
alerter (also deadman control): This device automatically applies the
brakes and stops the train if the engineer should suddenly become
incapacitated. If one of the controls in the locomotive (throttle,
brake, or whistle) isn't touched in a specific length of time (usu-
ally 20 to 30 seconds), a horn will sound and a bright light will
flash inside the locomotive cab. The engineer then has five sec-
onds to respond before the brakes are automatically applied.
articulated car: Two or more railcars joined together to function as
one unit. Passengers can pass from one car to another without
opening and closing doors. The cars flex at each connection as
the train rounds curves.
A-unit (also called the lead unit): There can be three or four locomo-
tives pulling a long passenger or freight train. This term refers to
the first locomotive at the head of the train, usually the only one
with an engineer.
automatic-train-stop system (usually referred to as ATS): If a train passes
a signal calling for reduced speed and the engineer doesn't elec-
tronically acknowledge it, this system will stop the train auto-
matically. See also positive train control .
axle: The steel shaft on which the wheels of a railcar are mounted.
Unlike most other wheeled vehicles, the railcar wheels are welded
to the axle, which then revolves with the wheels. This is an extra