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 and two in Cardiff.
Over the August Bank Holiday weekend 2016 control of the signals between Westerleigh Junction and Pilning was switched over to the Thames Valley Signalling Centre.
These signals now carry the prefix 'BL'.

The maximum line speed from London to Coalpit Heath is 125 mph (200 km/h); 90 mph (145 km/h) from Coalpit Heath to Newport; 90 mph (145 km/h) from Newport to east of Bridgend; 75 mph (120 km/h) from east of Bridgend to Swansea Loop North junction (with a small section of 100 mph (160 km/h) track through Pyle station); and 40 mph (65 km/h) from Swansea Loop North Junction to Swansea.
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 and two in Cardiff. Over the August Bank Holiday weekend 2016 control of the signals between Westerleigh Junction and Pilning was switched over to the Thames Valley Signalling Centre. These signals now carry the prefix 'BL'.
The maximum line speed from London to Coalpit Heath is 125 mph (200 km/h); 90 mph (145 km/h) from Coalpit Heath to Newport; 90 mph (145 km/h) from Newport to east of Bridgend; 75 mph (120 km/h) from east of Bridgend to Swansea Loop North junction (with a small section of 100 mph (160 km/h) track through Pyle station); and 40 mph (65 km/h) from Swansea Loop North Junction to Swansea.
South Wales Main Line