HOW IT ALL WORKS - All Aboard: The Complete North American Train Travel Guide - page 179

for passenger trains; the bottom number applies to freight trains.
There is an exact equivalent on many of our highways, where
there are sometimes two posted speed limits, one for passenger
cars and another for trucks. The principal difference is that signs
along a highway tell drivers what the speed limit is between that
exact spot and the next sign, while most speed limit signs on a
railroad tell the engineer how fast he should be traveling two
miles ahead. The reason for that difference, of course, is that it
can take that distance for the engineer to reduce his speed gradu-
ally from, say, 70 to 40 miles per hour. If the number shown on
the sign means the engineer can increase the speed of the train, he
may do so as soon as the entire train passes the sign. Train crews
often refer to these signs as “speed boards.” Just to confuse you,
on some railroads there are two kinds of speed limit signs: black
numbers on yellow signs mounted diagonally on the pole indicate
the speed limit two miles ahead; black numerals on white signs
mounted level indicate the speed limit at that spot.
Exceeding the posted speed limit is a big no-no in railroading.
Inspectors from the Federal Railway Administration are respon-
sible for enforcing speed limits and other safety rules, and they do
so by checking train speeds with radar guns. One or two speeding
offenses by an engineer, even a few miles an hour over the limit,
and he or she will be looking for a new career.
Crossing Ahead
This is a black “X” on a white sign and tells the engineer that
the tracks his train is on will be crossing another set of tracks up
ahead. (Just to confuse us, some railroads use this sign to warn
the engineer that there is a grade crossing ahead.)
Derail Signs
You'll often see these little signs, usually black letters on yellow
or orange signs, along sidings as you enter or leave rail yards or