Does anyone know of a source for how routing has evolved over the last... well, I'm not sure. Perhaps since the railway moved from a pattern of infrequent services over a rather more extensive infrastructure and started to aspire to, say, hourly clockface patterns of services on the network we use today.

Is guidance for passengers to the effect: 'If the National Rail site won't sell a ticket for it, then it's not a permitted route'?

An issue is that the National Rail site can give the impression that for some journeys that had an alternative that was rather longer than the arbitrary 'Up to 3 miles longer is a permitted route and anything else needs an explicit easement' rule, routing easements have somewhat contracted, which is not to the benefit of the intending traveller.

Mark

Well, services were improving to hourly or better and clockface in BR^▸ (British Rail(ways)) days, long before "any reasonable route" became "any permitted route" and the Routeing Guide came into existence.

The thread https://www.railforums.co.uk/threads/national-routeing-guide-update.99396/ starts in 2014, and the oldest I can find on the forum is from 2007: https://www.railforums.co.uk/threads/routeing-guide-easements.14305/

While these cover at-the-time changes, I don't know of any record of the cumulative effect of all the changes. I suspect that it would be difficult to research, especially as there are non-Guide changes that affect the choice of permitted routes. For example:

• Timetable changes that add or remove through services
• Some journeys not having an "any permitted" route option (only fares for specified routes)

Personal views ...

Many easements that I'm aware of have come into use because of a horrid infrequency of direct trains - journeys like from Cheltenham Spa and south thereof to Bromsgrove allowing double-backs, and journeys to and from Dilton Marsh doubling back st Westbury or Warminster.  Once direct hourly or better direct services are offered, the easement becomes developed out of making sense.   Open another platform at Pilning and the easement to double back to get there if arriving from / through Patchway (or noting the lines are bi-directional, run some services "wrong line") and you can clear away another easement.

"Any permitted" is a joke of a use of the English language.   It's a tautology and every ticket is permitted to be used on any route on which it is permitted - duh! ... Agreed that the permission may be "via Bath Spa" on a particular ticket but even when that's added to the ticket, it is still permitted to be used by any route on which it is permitted to be used!

I would favour a return of the term "any reasonable route" and to define that carefully to reduce any confusion. To be clear, "any reasonable route" would start with any train scheduled to get you to your destination, with intermediate changes if need be, ahead of any other route.   And an "any reasonable route" ticket should be always available.

That "any reasonable route" would overcome - for example - those very silly suggestions from booking engines (clue - it's called a booking engine because people want to book tickets) that tell you of a travel opportunity but then tell you "no tickets available".  Try Melksham to Chepstow.    Or try (but the booking engine does not offer it) Melksham to Bristol Parkway where much the fastest is a usually a single change at Swindon.

A slower route with fewer (or no) changes should be allowed to, and a slower route which moves the interchange station (s) between services that are fit for the less able - that includes disabled, but also those with heavy luggage, babes in prams, arms or tummies who are less able but not classified as disabled - and indeed allow anyone to make those extended changes.