WHAT ABOUT ALL THOSE FREIGHT TRAINS? - All Aboard: The Complete North American Train Travel Guide - page 200

tractive force . Horsepower is simply the common way of measur-
ing how powerful the locomotive is, which is exactly the way
we measure our cars' engines. Another term for tractive force ,
and certainly more familiar to the layperson, would be traction .
Whether it's an automobile or a locomotive, all that power can
become almost useless if the wheels start slipping. That's what
tractive force is all about, and that's why railroad locomotives
are so heavy—to increase the tractive force and keep the wheels
from slipping on the rails.
Depending on the gross weight of a train, additional “helper”
locomotives are usually added to provide more horsepower and
more tractive force. The train's weight is determined by the num-
ber of cars in the consist and what's in those cars. Obviously,
100 cars loaded with coal will weigh much more than 100 cars
loaded with wood chips, but the average capacity of a freight car
is about 89 tons.
Passenger trains weigh much less than freight trains, but they
need to go faster. That's why passenger locomotives can afford
to sacrifice tractive force in favor of more horsepower. It's an
oversimplification, but generally speaking freight locomotives
are designed to pull very heavy weights at relatively slow speeds,
while passenger locomotives are designed to pull less weight but
go faster. Until Amtrak's P-40s entered service, most of the loco-
motives used to pull passenger trains were freight engines modi-
fied to operate at higher speeds.
Freight Cars
There are about 1.4 million freight cars of all types traveling back
and forth across the country. About a third of those are owned by
the Class I railroads; the rest are owned by independent railroad
car companies, the shippers themselves, and the small regional