Equally valuable improvements were made in cars, both for passengers and freight. Instead of the four-wheeled English car, which on a rough track dances along on three wheels, we owe to Ross Winans, of Baltimore, the application of a pair of four-wheeled swivelling trucks, one under each end of the car, thus enabling it to accommodate itself to the inequalities of a rough track and to follow its locomotive around the sharpest curves. There are, on our main lines, curves of less than 300 feet radius, while, on the Manhattan Elevated, the largest passenger traffic in the world is conducted around curves of less than 100 feet radius. There are few curves of less than 1,000 feet radius on European railways. [Illustration: A Steep Grade on a Mountain Railroad.] The climbing capabilities of a locomotive upon smooth rails were not known until, in 1852, Mr. B. H. Latrobe, Chief Engineer of the Baltimore and Ohio Railroad, tried a temporary zigzag gradient of 10 per cent.--that is 10 feet rise in 100 feet length, or 528 feet per mile--over a hill about two miles long, through which the Kingwood Tunnel was being excavated. A locomotive weighing 28 tons on its drivers took one car weighing 15 tons over this line in safety. It was worked for passenger traffic for six months. This daring feat has never been equalled. Trains go over 4 per cent. gradients on the Colorado system, and there is one short line, used to bring ore to the Pueblo furnaces, which is worked by locomotives over a 7 per cent. grade. These are believed to be the steepest grades worked by ordinary locomotives on smooth rails. Another American invention is the switchback. By this plan the length of line required to ease the gradient is obtained by running backward and forward in a zigzag course, instead of going straight up the mountain. As a full stop has to be made at the end of every piece of line, there is no danger of the train running away from its brakes. This device was first used among the hills of Pennsylvania over forty years ago, to lower coal cars down into the Nesquehoning Valley. It was afterwards used on the Callao, Lima, and Oroya Railroad in Peru, by American engineers, with extraordinary daring and skill. It was employed to carry the temporary tracks of the Cascade Division of the Northern Pacific Railroad over the "Stampede" Pass, with grades of 297 feet per mile, while a tunnel 9,850 feet long was being driven through the mountains. [Illustration: A Switchback.]