Swinging arm bogie (SAB), from the APT-E trailer cars, British Railways, 1972.

The APT-E is a semi-permanently coupled articulated train. Unlike the power cars, which have dedicated leading bogies with traction motors, the unpowered trailer cars in the APT-E are suspended at either end by a common bogie. This has the advantage of reducing the number of bogies and wheels as well as reducing the overall weight of the train, while making for a more comfortable ride and potentially increasing seating capacity.

A Swinging Arm Bogie (SAB) uses a steering beam to link both cars together and is part of the bogie steering. This was attached to two swinging arms at either end of the bogie, which were themselves attached to the frames housing the wheel axles. Between these is a third frame carrying part of the pneumatic suspension. On testing in 1972, this bogie design suffered from ‘hunting’ oscillation and was unstable at high speed, and was replaced by an unpowered version of the bogies used under the power cars, designated E1T. This SAB is the sole-survivor and was used on the experimental POP Train used to test the APT’s suspension, tilt mechanism and hydrokinetic braking system.