WHO’S IN CHARGE HERE? - All Aboard: The Complete North American Train Travel Guide - page 110

To start the train, the engineer opens the throttle, which
causes the diesel motors to run faster, generating more electricity
and increasing the flow of power to the traction motors on each
of the locomotive's axles. As that happens, most engineers lean
out of the window and look straight down at the ground. In that
way, they can tell the instant the train starts to move and can
adjust the throttle accordingly to keep the wheels from slipping.
That's a real no-no, because it can cause a jerk to be felt through-
out the train. Passenger-train engineers pride themselves on their
smooth starts, so to start with a small jerk is mildly embarrassing.
But there are potentially more serious consequences to a rough
start. If the jolt is strong enough, it can cause passengers or crew
to momentarily lose their balance and even fall—an extremely
rare occurrence, by the way.
Freight trains are much longer than passenger trains, and
there is a lot more slack, or “play,” built into the coupling mecha-
nism hooking each car together. As a freight train starts moving,
the slack is taken up in one car after another down the length of
the train. In years past, brakemen in the cabooses would hear the
jolt coming toward them and brace for it. As the slack is taken up
in car after car, it builds in intensity; by the time it gets to the last
car of a long train, it can literally knock you off your feet.
As strange as it may seem, freight engineers must handle their
trains very gently. Taking up slack abruptly while a freight train is
rolling can literally snap a coupling and break the train in two. In
fact, to prevent massive derailments, the knuckle in the coupling
device is deliberately designed to be the weakest link in the chain
and to break under severe stress. How many times one “breaks”
a train is one way of measuring the performance and abilities
of a freight engineer. Sometimes it happens because the engineer
caused it, but other times a knuckle gives way from metal fatigue
or because it's old and rusted. When that happens, the engineer
will replace the defective knuckle (spares are carried in freight