The direction-determining characteristic of rails also works as a severe limitation in
railroad operations. Several trains operating on the same track must proceed in the same
direction and at the same speed to avoid collisions. The turnout was developed to alleviate this
inflexibility. Systems of turnouts make passing possible and provide a means for trains to leave
one track and proceed to diverse destinations over other paths. Yards, interlockings, passing
sidings, crossovers, and elaborate signaling mechanisms depend upon or are associated closely
with these turnouts. Such installations have been made at great expense to achieve a flexibility in
traffic handling that is, nevertheless, inferior to that of many other modes. Here is a
fundamental characteristic of railroads that appears over and over again: railroads depend upon
expensive fixed installations to exploit their basic advantage--moving large numbers of persons
and great quantities of freight easily and speedily. Railroads must handle a large volume of
traffic to justify the expense.
Physical analysis shows that the coefficient of friction between rails and smooth wheels is
low. Hence, less energy is expended in moving any particular load by rail. Lower energy
expenditures result in lower fuel costs. However, the friction between rail and wheel is the
means of transmitting traction or braking forces. Since this friction is small, the tractive effort
that may be applied is low. Trains are, therefore, limited to light grades even though rail lines
must cross uneven or mountainous terrain. The difficulty is solved by judicious use of cuts, fills,
loops, switchbacks, tunnels, and bridges. Again, railroads depend on expensive stationary
construction to operate.
One of the principal problems railroads must solve is that of determining how far to go in
reducing operating difficulties and expense by increasing the elaborateness of the physical plant.
Obviously, a point is reached beyond which the expense of improving the stationary plant cannot
be justified from the standpoint of ease of operations. Ordinarily, heavy traffic lines not only
justify but demand refinement of fixed facilities. But a low traffic level may not support a rail
line, or, at best, it may justify only a simple, unrefined plant.
You should now understand one of the foremost principles of railroading in general and
of maintenance of way in particular. It is this: any problem of railway maintenance or
construction must be solved in relation to the density and type of traffic that the rail line carries
or is expected to carry. This principle underlies all discussion in this text. To make it clearer,
here are two examples.