HOW IT ALL WORKS - All Aboard: The Complete North American Train Travel Guide - page 160

For example, it sounds simple to say that the California
Zephyr runs between Chicago and the San Francisco area. But
remember that the train runs every day in each direction, and
it's a two-night trip. So, at any given moment, Amtrak needs
six complete trains to operate the California Zephyr: two are
en route heading west, another two are heading east, and two
more are getting ready to depart at either end of the route. Each
of those six trains requires nine cars and two (sometimes three)
locomotives. That means, at a minimum, Amtrak needs 54 rail-
cars and a dozen locomotives just to run that one long-distance
train. Then, of course, there are three other western trains that
run two-night trips: the Sunset Limited, the Southwest Chief,
and the Empire Builder. In addition, there are eleven other trains
that operate over routes that require one overnight to complete.
Amtrak also has what they call “protect locomotives” located in
strategic places around the country and ready to be pressed into
emergency service—there are some in Denver, for example, in
case one of the Zephyr's locomotives should fail.
There are some obvious differences between passenger and
freight operations, but the basic technology and operating pro-
cedures are pretty much the same—whether it's an Amtrak train
carrying 300 people from Seattle to Los Angeles or a freight
train hauling coal from a Wyoming mine to a power plant near
Chicago.
The Track
When you stop and think about it, it's the track that makes rail
travel different from every other form of transportation. Rail-
roads spend a lot of time and money on the construction and
maintenance of their track. It's important too, because poor
track means trains must run at slower speeds; that in turn means
reduced revenues for the railroad.