The 06:00 weekday Portsmouth Harbour to Cardiff Central has ten minutes at Westbury and Bristol Temple Meads. The train used to attach and divide at these locations but this is no longer the case
Any other passenger services with a ridiculously long dwell time?
(Of note is the 06:00 1F06 bizarrely omits Avoncliff whilst stopping everywhere else between Warminster and Bristol)
I used to catch the 0600 Portsmouth-Cardiff (0800 from Trowbridge) a few years ago and it had a terrible punctuality record - it hasn’t always had that dwell time at Westbury, even when they used to attach a 153 to the rear of a 3 car 158 which can’t have helped. Probably could run a bit earlier again now there is no attaching but it would clash with a London - Bristol IET at Bath these days. It probably doesn’t need to stop at Avoncliff because Avoncliff gets two trains an hour stopping in the morning peak towards Bath and Bristol, and is quieter than Freshford.
There are some Westbury - Gloucester services that have very long waits at Temple Meads. The 15:50 Westbury to Gloucester has a 22 minute wait at Temple Meads which will be rather off-putting for anyone who wants to go from Bath to Gloucester! Likewise some local trains at Dawlish Warren have long waits there.
Assuming the train a) is booked turbo and b) was strengthened between Westbury and Bristol- that’s probably why the “local door only” Avoncliff is omitted. I doubt local door and non-inter-unit gangways are considered compatible.
GWR have often stopped at short platforms with no gangways - It happens at Avoncliff, Thornford, Chetnole and Dilton Marsh but would’ve happened historically with St James’ Park, the Barnstaple line, Patchway (when 10 car IETs stop there during engineering works) and probably more.
For Avoncliff, the guard makes an announcement at the previous station advising passengers in the rear train to alight, walk down the platform and join the front unit.