These are extracts from the GWR website pages dealing with the closure of the Severn Tunnel:
Severn Tunnel Modernisation - September/ October 2016
September/ October 2016
From Monday 12 September to Friday 21 October 2016, the Severn Tunnel will be closed. This means no trains will run between Patchway and the Severn Tunnel Junction; with London services diverted and replacement buses between Newport and Bristol Parkway.
Weekdays
Between London Paddington and South Wales
High Speed services from London Paddington to South Wales will be diverted between Swindon and Newport; increasing journey times by approximately 30 minutes. Trains will still run between London Paddington and Bristol Parkway.
Between Cardiff Central and Portsmouth Harbour
Trains will run between Bristol Parkway and Portsmouth Harbour
Between Cardiff Central and Taunton
Trains will run between Bristol Parkway and Taunton.
Replacement buses
Replacement buses will run between Severn Tunnel Junction and Bristol Parkway, and between Bristol Parkway and Newport.
Any additional routes will be confirmed nearer to the time.
Weekends
17 to 18 September and 15 to 16 October
The railway from Bristol Parkway through to Cardiff Central will be closed and replacement buses will be in operation between these stations.
Trains will run normally between London Paddington and Bristol Parkway; with a shuttle train service operating between Cardiff Central and Swansea.
24 to 25 September, 1 to 2 October and 8 to 9 October
In addition to the Severn Tunnel closure, trains wont run between Swindon and Bristol Parkway. This wont affect South Wales services as they are being diverted away from this section. Replacement buses will run between Swindon and Bristol Parkway.
Further works affecting services into South Wales
3 to 4 and 10 to 11 September
Trains will be diverted between Cardiff Central and Bridgend increasing journey times by approximately 30 minutes.
Sunday 11 September
The railway between Bath Spa and Westbury will be closed. Trains between Cardiff Central and Portsmouth Harbour will be diverted via Swindon; increasing journey times by approximately 35 minutes. Replacement buses will be available between Westbury and Bath Spa.
22 to 23 and 29 to 30 October; 5 to 6 and 12 to 13 November
The railway between Didcot Parkway and Swindon will be closed all weekend (extending to Bristol Parkway on Sundays). Trains to South Wales will be diverted increasing journey times by approximately 45 minutes.
Revised timetables
The best way to find details of train times and replacement services is to use our online journey planner.
Or, if you prefer you can download our PDF timetables:
London to South Wales
Cardiff to the South Coast
Here's a link to this page:
https://www.gwr.com/travel-updates/planned-engineering/severn-tunnel
On a quick check over a period of about four or five hours between London and Cardiff, it looks as if the diverted trains will
not call at Gloucester.