The large travelling access platforms fitted to the underside of the Second Severn Crossing Bridge and their rails were due to be refurbished. Using the platforms themselves to carry out this work would have been a complex and laborious task, partly because of their slow speeds and partly due to the fact that they were to be removed as part of the refurbishment. What was needed was a secondary, speedy access system to allow the workers to reach each section quickly and safely, whilst the platforms were removed by large cranes, and afterwards.
SCX Special Projects designed and installed a Rapid Access Train (RAT) which would run along one of the platforms’ rails, and allow four maintenance workers and their equipment to quickly and safely reach the three access gantries, two pylons and the ten stations (including one at each abutment) of the bridge.
The diesel-powered Rapid Access Train (RAT) consists of a ‘tractor’ (the engine compartment), the cabin which seats up to four persons and the trailer for carrying the equipment. The entire system has a SWL capacity of 2.4tonnes (400kg in the cabin and 2tonnes on the trailer).
As it enters the station, the RAT is slowed and steadily brought to a halt by a pre-limit switch. When it approaches or leaves the station, automatic gates open to allow it to pass and then close again to protect the edge of the platforms.
With a maximum speed of 1.5m/s along most of the track, the RAT is automatically restricted to 1.0m/s in pre-determined sections, including before and inside the stations, around the joint areas and inside the pylon stations.
Control desks face in either direction, complete with joysticks to control speed and a dead-man switch to bring the train to a complete halt in the event of an emergency. In case of a power failure, an emergency override recovery drive system allows the RAT to be driven to the nearest station on a backup battery supply.
A 1tonne SWL chain hoist loads large items into the RAT through large front doors, whilst lightweight items can be loaded by hand through a lateral sliding door. The RAT and this lifting equipment are programmed so that they cannot be operated simultaneously.
