HOW IT ALL WORKS - All Aboard: The Complete North American Train Travel Guide - page 165

Here's something else unique about the way a train works:
the wheels of a railcar are welded to the axles and the entire unit
turns. You'll notice it immediately if you look carefully at the
wheels of a train rolling by at a slow speed. It's done that way
to ensure that the wheels stay on the rails, and it provides better
distribution of the immense weight of the railcar.
It All Starts with the Locomotive
The proper term for this monster is indeed locomotive , but rail-
road people also use the terms engine and sometimes moto . Like
ships, a locomotive is always spoken of as though it were female.
When it rolls off the production line, every locomotive is
assigned a number that is unique all across the country. No two
have same number, regardless of which railroad may own them.
When speaking about a modern railroad locomotive, most
people refer to it as a “diesel.” Well, they're half right. Techni-
cally, it's a diesel-electric locomotive. Its diesel motors (there can
be several in one locomotive) are not actually used to move the
train but to power electric alternators that generate direct cur-
rent (DC), which is sent to traction motors on each of the loco-
motive's axles. That's what makes the train go. In addition to
powering the traction motors, a locomotive's generators run fans
for removing heat and the air compressor for the braking system,
and they provide electricity for the rest of the train.
There are also all-electric locomotives. Instead of producing
their own electricity, they draw it directly from an overhead wire,
called a catenary . These locomotives are much more powerful
than the diesel-electrics, and they're faster too.
In any discussion of railroad locomotives, traction is the key
word. That's why locomotives are heavy—deliberately weighted—
to provide as much traction as possible. Traditionally, traction
motors have used DC, but that's changing. Some newer locomo-