HOW IT ALL WORKS - All Aboard: The Complete North American Train Travel Guide - page 162

then both are fastened to the ties with track spikes, which are
really just huge nails. Tie plates are embedded in the concrete ties
when they're formed, and the rails are fastened to the ties with
heavy metal clips. There are minor variations, but generally that's
the way all track is laid.
The two rails are laid exactly four feet, eight-and-a-half
inches apart, which is known as standard gauge. The importance
of establishing a standard width for track is obvious: It means an
individual freight car can be hauled virtually anywhere in North
or Central America. Parallel tracks are laid 14 feet apart—that's
the distance from the center of one track to the center of the
adjacent track.
There are a number of narrow-gauge railroads in this coun-
try. These tracks are just three feet apart to accommodate smaller
locomotives and cars designed for operating on steep grades and
around tight curves in mountainous terrain. Most of these nar-
row-gauge railroads have become tourist attractions and provide
excellent opportunities to experience railroad operations typi-
cal of another era—not to mention a chance to view spectacular
scenery up close. (You'll find links to a list of scenic excursion
trains in the back of this topic.)
Traditional 39-foot rails are bolted together using a metal plate. See the small space between
the rails? That's what produces the clickety-clack sound. Photo courtesy of the author