Tilting trains

A tilting train is a train that has a mechanism enabling increased speed on regular rail tracks. As a train (or other vehicle) rounds a curve at speed, objects inside the train experience inertia. This can cause packages to slide about or seated passengers to feel squashed by the outboard armrest due to its centripetal force, and standing passengers to lose their balance. Tilting trains are designed to counteract this discomfort. In a curve to the left, the train tilts to the left to compensate for the g-force push to the right, and vice versa. The train may be constructed such that inertial forces cause the tilting (passive tilt), or it may have a computer-controlled power mechanism (active tilt).

The first tilting train in regular public service was the 381 series electric multiple unit train operated by Japanese National Railways (JNR), which entered revenue service on 10 July 1973 on the Shinano limited express between Nagoya and Nagano on the Chūō Main Line. This technology was not fully implemented worldwide, as the marginally increased curve speeds did not justify the extra expense and technology in many cases. The British Advanced Passenger Train (operational from 1984 to 1985) was the first to implement active tilt successfully, enabling significantly increased speeds on tight rail curves. Active tilting is the mechanism most widely used today.

Aeroplanes and bicycles tilt inwards when cornering, but automobiles and trains cannot do this on their own. Vehicles with high centres of gravity rounding sharp curves at high speeds may topple over. To make their turns easier, the outer edge of a roadway of a high-speed highway or outer rail of a railway may be canted (raised) upward around the curve. The combination of tilt and centrifugal force combines to produce an effective acceleration that is down through the floor, reducing or eliminating any sideways component.

The particular angle of tilt ("superelevation") is determined by the intended vehicle speed — higher speeds require more banking. But with a growing desire in the 1960s and 1970s to build high-speed rail networks, a problem arose: the amount of tilt appropriate for high-speed trains would be over-tilted for lower-speed local passenger and freight trains sharing the lines. Japan's early bullet train efforts of the 1960s avoided this problem by laying all-new lines as part of a re-gauging effort, and France's TGV followed the same pattern. Other operators did not have this luxury and were generally limited to much lower speeds.

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