In message , at 06:58:58 on Thu, 22
Jan 2015, Clive D. W. Feather remarked:
It's a Route-specific ticket "via Taunton". And on the day I want to
travel there's engineering work, and the train is being diverted away
from Taunton.

In general engineering diversions are ignored for routeing purposes.

It would help if the page for Future Engineering Works mentioned ticket
validity, as well as the changes being made that day. eg for the trip I
describe:

"Trains between London Paddington and Plymouth / Penzance normally
routed via Newbury will be diverted (not calling at Taunton and Tiverton
Parkway) and run to an amended timetable. Journey times may be extended
by up to 60 minutes." ... adding "Tickets routed 'via Taunton' are
valid on these trains".

Or do they think it's 'obvious'? The ticket office staff here are
adamant that it's obvious the ticket *wouldn't* be valid

That's not to say they are suggesting that travel isn't permitted at
all, but they claim it's necessary to get the XC train which terminates
at Taunton, then the replacement bus to Bristol, and FGW to Paddington.

Though if someone boarded a train with a ticket only valid on the
diversion route it ought to be accepted.

Ticket-selling sites seem a bit confused. East Coast, for example, won't
sell the "via Honiton" ticket on the service, but neither will it sell
the "via Taunton"! What it will do is sell an Advance.
--
Roland Perry