Infrastructure
There are four tracks from Severn Tunnel Junction through Newport to Cardiff Central, with two tracks on the remaining sections. Multiple-aspect signals are controlled from several power signal boxes including Swindon, Bristol, Newport and Cardiff.
The maximum line speed from London to the Severn Tunnel is 125 mph (201 km/h); 90 mph (140 km/h) from Severn Tunnel to Newport; 75 mph (121 km/h) from Newport to east of Bridgend; 75 mph (121 km/h) from east of Bridgend to Swansea Loop North junction; and 40 mph (64 km/h) from Swansea Loop North Junction to Swansea.
Read more about this topic: South Wales Main Line