Imagine yourself seated in a luxurious air-conditioned dining car enjoying a delicious breakfast of bacon and eggs, toast and milk, while your train speeds merrily along hundreds of feet underground. This is happening every day in parts of our country where railway tunnels pierce great mountain ranges.

Many passengers entering and leaving New York City pass directly under huge ocean liners steaming to and from their piers on the North River. In Washington passenger trains pass through a tunnel that carries them almost directly beneath the Senate Office Building, the Supreme Court Building and the Library of Congress.

The tunnel ranks with the bridge among the most interesting features of the railroad. There are more than 1,500 railroad tunnels in the United States, ranging in length from a hundred feet or less to several miles—carrying railway traffic beneath city streets and skyscrapers, under busy rivers and harbors, through hills and beneath rugged snowcapped mountains.

The tunnel is the direct opposite of the bridge. The bridge carries the railroad above the earth's surface; the tunnel carries it beneath the earth's surface. Both are difficult and expensive to build; therefore, both are avoided whenever possible. There are many instances where tunnels are necessary to provide a more direct route between two points than would otherwise be possible, to enable the railroad to traverse mountains at fairly level grades, or to gain favorable entrances to large cities and obtain adequate terminal areas in those cities without seriously disturbing surface traffic.

The first railroad tunnel in America was built in 1833 to carry a railroad line through the Allegheny Mountains in Pennsylvania. In the years that followed many other tunnels were opened.

All of them were eclipsed by the Hoosac Tunnel, more than 25,000 feet in length, completed in 1875, to carry a railroad line through Hoosac Mountain in western Massachusetts. This tunnel is still in use.

Since 1900 two longer railroad tunnels have been built in this country. One of these is the Moffat Tunnel, carrying a railway line for more that 32,000 feet through James Peak in Colorado. This tunnel was completed in 1928. At its highest point, Moffat Tunnel is 9,257 feet above sea level.

The other tunnel, the longest in the Western Hemisphere, is the Cascade Tunnel, 41,152 feet in length, piercing the Cascade Mountains in the State of Washington. It was completed in 1929.

The most stupendous tunnel project in the history of American railroading, from the standpoint of total cost, was the twin-tube tunnel under the Hudson River and the four-tube tunnel under the East River in New York, together with terminals and yards under the rivers and under the teeming city itself. This project, completed in 1910, employed many thousands of workmen and great quantities of equipment, tools and machinery. It cost in the neighborhood of 113 million dollars.

The construction of a great railway tunnel calls for engineering skill of the highest order, for an error in reckoning might prove extremely costly. In the construction of the Cascade Tunnel boring through solid rock was carried on from both the eastern and the western portals. When the construction forces met, each nearly four miles from their respective portals, the engineers found that they were only a fraction of a foot out of perfect alignment. This small error in reckoning was easily rectified.

Many tunnels are bored through solid rock by the aid of pneumatic and electric-drills and explosives. A temporary railway track is extended from the portal into the bore as fast as the drilling proceeds, and this is used to carry off loose rock and boring dust. It is also used to transport workmen, equipment and materials. Electric fans or ventilator pipes keep fresh air circulating in the bore. Pumps draw off seepage water.

Some tunnels are built for one track only, others are built for two or more tracks, depending upon anticipated traffic and other operating conditions. Many tunnels are lined with concrete, brick or timber, or a combination of these materials, and many are made waterproof to prevent seepage. Linings strengthen the walls and ceiling and increase the safety of train operations.

Many railroad tunnels are electrified, and trains are operated through them by electric locomotives.