Infrastructure
Between London and Didcot there are four tracks, grouped by speed with the "relief" lines on the north side of the "main" lines. Most smaller stations only have platforms in use on the relief lines. Between Didcot and Royal Wootton Bassett there are a series of passing loops lines to allow fast trains to overtake slower ones. This section is also signalled for bi-directional running on each line but this facility is usually only used during engineering working or when there is significant disruption to traffic in one direction.
The line is currently not electrified except for a short 12 miles section of electrified 25 kV AC overhead wires between Paddington and Airport Junction (the junction with the line to London Heathrow Airport near Hayes).
The line speed is 125 miles per hour (201 km/h). The relief lines from Paddington to Didcot are currently limited to 90 miles per hour (140 km/h) as far as Reading, and then 100 miles per hour (160 km/h) to Didcot. Lower restrictions apply at various locations. It is one of only two Network Rail-owned lines to be equipped with the Automatic Train Protection (ATP) system, the other being the Chiltern Main Line.
Read more about this topic: Great Western Main Line