Published by Australian Academy of Science
Looking down the track at very fast trains Box 1 | The centrifugal effect and tilt trains

Think about what happens if you are travelling in a car that takes a corner at high speed. You are forced against the car door and the drink in your lap overturns. These things occur because momentum keeps you and the drink-bottle moving in a straight line while the car itself has started to turn. This phenomenon is known as a centrifugal effect.

Similar things will happen if a train takes a bend too fast. Standing passengers will lurch over and luggage may fall down from racks. If the centrifugal effect is strong enough (and it has to be very strong), the train itself might fall off the tracks.

Smoothing out the bends

Train companies wish to avoid disturbing their customers with centrifugal effects, so they adopt strategies for 'smoothing' out the bends and corners that a train inevitably has to take.

The centrifugal effect is proportional to the square of the velocity of the train and inversely proportional to the radius of the bend in the track. This means that if you want to double the velocity of the train but keep centrifugal effects constant, you must increase the radius of the bend four-fold.

Tilting the train

Another strategy for coping with centrifugal effects is to tilt the train itself – this is used, for example, by the Swedish X2000 series that can travel up to 200 kilometres per hour. If the train is tilted, and with it all the tables, seats and passengers, then the tendency for everything to slide down this slope can exactly balance the centrifugal tendency for them to move towards the outer wall of the carriage, and everything feels normal.

The actual tilting of the train is done by pivoting individual carriages on their bogies using pistons. The pistons are controlled by computer and move up or down depending on the strength of centrifugal effect. By counteracting the centrifugal effect in this way, 'tilt' trains can travel up to 40 per cent faster around curves than conventional trains.

Tilting technology was tried in Australia on the Sydney-Canberra line for a 2-month period in 1995. A trip that usually takes 4 hours and 5 minutes on the XPT was reduced to 3 hours and 25 minutes, a timesaving of about 16 per cent. A new tilt train service is to be introduced later this year between Brisbane and Rockhampton. The service should reduce travelling time from 9.5 hours to 7 hours.

Related sites

• Force: Centripetal vs centrifugal (Public Broadcasting Service and Turner Adventure Learning)

• Tilting trains (Oliver Keating)

Other boxes

Box 2. How the maglev works