Many London-Exeter-Plymouth-Penzance services will be diverted at Castle Cary via Yeovil and Honiton but capacity limits on that route mean that this is not possible for all trains while pathing restrictions and lower line speeds will extend journey times by as much as 65 minutes. To accommodate this, those trains being diverted will start their journeys earlier from the west - for example the up Golden Hind will depart Penzance at 04.00) but down services will generally leave Paddington at around the usual times but with later arrival times at their destinations. This arrangement is to try to ensure that key services arrive at and depart from Reading and Paddington at times close to normal, thus allowing for business meetings, flight connections, etc. The sleeper trains will also be diverted via Yeovil but without significant alteration to their journey times which are already generous.
No through trains will operate between Paignton and Paddington. Passengers will need to change at Exeter St David's into and out of connecting services.
Some trains will continue to serve Taunton from London, terminating there. All CrossCountry trains will also terminate and start from Taunton. This means, of course, that passengers from further west will need to use the road transport from Exeter to Taunton to connect into CrossCountry services. There will also be some alterations to times between Taunton and Bristol during this period.
Some additional changes will occur between Exeter and Penzance, notably concerning those local services along the main line which normally originate from or continue to Bristol and beyond. Branch line times for the Gunnislake and Newquay branches (but not, generally, other branch lines) will also be adjusted to try to maintain main-line re-timed connections.
Note that the above is only an outline summary. Details will be on the FGW and National Rail web sites and the full timetable will be published by 9 December. Bookings via the internet and through ticket offices will automatically take the changed times and connections into account. This is clearly a major disruption not only to passenger services but also for freight (sadly not much of that) and for train operators who still have to get crews to their trains and trains to depots without breaching working hours or train servicing requirements. While the planning of the engineering work is very precise and the track must be handed back in time for the normal service to resume on Monday 10 February, this could be a challenging task if adverse weather such as experienced in recent winters occurs again during the possession. Many thanks to Derek Buttivant for advising us of this press release.