HOW IT ALL WORKS - All Aboard: The Complete North American Train Travel Guide - page 171

the wheels, use it to hold them away from the wheels. In that
manner, the brakes would automatically be applied if the system
failed for any reason, and the train would stop. There have been
frequent changes and improvements ever since, but the basic
principle behind the air brake still works—and it's still the way
trains are stopped.
There is another braking system worth mentioning. Dynamic
braking is used to very gradually slow the train without applying
the air brakes. In the simplest terms, instead of sending electric-
ity to the traction motors to move the train, the whole process
is reversed. The turning wheels are used to power the traction
motors and generate electricity. The resulting drag causes the
train to slow gradually. That electricity has to be used in some
way, so it's put to work operating blowers that cool the traction
motors. Any excess is dissipated through giant grids on the roof
of the locomotive, like the coils on top of your electric stove.
A rough— very rough—equivalent is slowing your family car by
slipping it into second gear.
As a passenger, you can frequently tell whether the engineer
is using the dynamic brakes or the air brakes. For one thing, air
brakes slow the train much more rapidly. You can also hear them
quite clearly—not the escaping air pressure but the sound of the
brake shoes being applied to the wheels beneath your car. There
is no noticeable sound when the dynamic brakes are applied,
but you can usually feel a very slight bump just before the train
begins losing speed—it's the slack between the engine and the
baggage cars being compressed.
“Spotting” the Train
The tricky part to stopping a passenger train is more the where
than the how . The object is to stop the train at just the right spot
so that passengers waiting on the platform can step right into