Railroads

3912

Railroads

point switches if the train passes over the
switch points before passing the frog, and
trailing-point switches if the train passes the
frog before the switch points.

Grades,—The grade, or rate of ascent or
descent, may be expressed in the number of
feet of rise to the mile or, preferably, as a per
cent. Thus, a grade rising r ft.in 100 ft. meas-
ured along the track is a i per cent, grade or
52.8 per mile. Two per cent, is considered a
heavy grade, although some of the best rail-
roads of the country^ operating through the
AUeghanies or the Rockies, have overcome
short distances of grades of 2.5 per cent, or
more by means of reducing the train load or
using helper engines. Grades as high as 4 or 5
per cent, can be operated with the usual type
of locomotive, hauling greatly reduced loads.
For steeper grades geared locomotives or rack
railroads are employed. There are several such
rack railroads in the Andes regions of Chile
climbing grades as high as 6 per cent.

Curves.—Changes in the direction of a rail-
road line are made by joining the tangents
with a curve. The curves are arcs of a circle
for the greater part of their length, tapering
off to the tangents by transition curves, as
explained below. Curves may be simple,
compound, or reverse. They are designated
as to their sharpness by the number of
circular degrees subtended at the center by
an arc in the center line of track whose
chord is 100 ft. long. A train in rounding a
curve tends to lean outwards, due to centri-
fugal force acting on the center of gravity of
each car above the rails or point of support.
To overcome this, the outer rail is elevated,
causing the train to cant inward. A common
rule is to elevate the outer rail $4 in. per de-
gree of curvature and add j£ in., the maxi-
mum in any case to be 6 in.

Tunnels.—Tunnels are principally confined
to mountain roads, where their use may save
many miles of detours or an appreciable sav-
ing in the length and steepness of grade re-
quired to cross a mountain range. An open
cut is preferable if economically feasible. In
hard, firm rock tunnels are not lined, but in
soft loose rock or earth it is necessary to line
them with some permanent structure. Brick
or stone masonry or concrete is used. At the
present time the longest railroad tunnel in the
United States is the Cascade Tunnel used by
the Great Northern in crossing the Cascade
Range in the State of Washington. It is about
7.78 miles long. The electrically operated and
recently enlarged Hoosac Tunnel of the Bos-

ton & Maine in Western Massachusetts, 4?4
miles, was until recently the longest tunnel
in the United States and is still the longest
double-track tunnel. The longest tunnel in
the world is the Simplon Tunnel in the Alps,
which is 12^/2 miles long.

In recent years a number of under-water
tunnels have been built for railroad service
The most noteworthy are those of the Penn-
sylvania Railroad, built in connection with
that company's big passenger terminal de-
velopment at New York. These are six in
number—two under the Hudson or North
River between New Jersey and Manhattan
and four under the East River, connecting
with the yards on Long Island and giving
the Long Island Railroad, a subsidiary of
the Pennsylvania, its entrance into the Penn-
sylvania terminal. Prior to the construction
of these tunnels the Pennsylvania carried its
traffic into New York by means of ferry
boats. Another tunnel of importance is that
of the Michigan Central (now New York
Central) at Detroit. The building of this
tunnel overcame the necessity of using a car
ferry. All of the trains through these sev-
eral subaqueous tunnels are electrically oper-
ated. (See TUNNELS AND TUNNELING.)

Bridges.—One of the most noteworthy
tendencies of railroad development in the
United States for many years has been the
rapid replacement of timber or masonry
bridge structures by concrete or steel in con-
sequence of the use of larger locomotives and
heavier cars. For small openings reinforced
concrete, corrugated iron pipe or tile drains
are put in and for brooks or small creeks
larger openings, girder spans, cantilever or
steel arch structures are used or possibly slab
or arch concrete bridges, depending upon
the conditions to be met,

Freight Cars.—The distinguishing feature
of the freight cars used on the railroads of
the United States and Canada is their high
capacity and the tendency towards the use
of cars of still larger capacity. The ordinary
box car has a capacity of 40 to 55 ^ons and
the ordinary coal car of 50 to 55 tons. Large
numbers of coal and ore cars of 70, 75 and
even roo tons capacity have been built, and
the roads serving the West Virginia coal dis-
tricts have cars of 120 tons capacity. The
American freight car is usually mounted on
two swivelling four-wheel trucks or bogies.
American freight cars are divided into the
following principal classes: box or covered
cars; gondola cars, having sides but no