I worked in the Passenger Commercial area of BR in the early 1970s.
The arrangements as I remember them were as follows.
Tickets were available generally "by any reasonable route", which of course then was somewhat easier to identify of course.
Firstly we need to look at the fares structure. Fares at that time were primarily mileage-based, but certain journeys were subject to what was known as "Selective Pricing", that is the mileage was the basis for the fare but then there was an add-on to artifically raise the fare to reflect the level and type of service, such add-ons were not massive, however to get around the situation of a journey such as say Levenshulme to Euston or Stechford to Euston which would be cheaper by a considerable margin, the SPM would instruct to use the fare from Piccadilly or New St as appropriate.
Not all fares were shown in the SPM. The fares shown there were based upon traffic flows and these were varied as required by discussion between the Divisional Passenger Manager within each Divisional Manager's Office (DMO), and the regional General Manager's office (GMO).
All local fares were the responsibility of the DMO and were published separately in Divisional Fares Manuals. Copies of these were sent to all of the GMOs, where they were then in turn distributed to the relevant DMOs, the Areas and major booking offices. Obviously onlt relevant DFMs were circulated within each Region, for example London had everything, whereas Birmingham had only these immediately surrounding (London/Stoke/Gloucester) whilst Leeds for example would have Newcastle/Doncaster/Manchester/Preston.
The DFMs were generally matrix or triangular based tables covering a route or a number of routes. Some stations had each local destination shown, plus major stations depending again upon the level of demand.
Any journey from insignificant station A to insignificant station D would pretty much always include an available fare from the the SPM, and by adding on the three elements, the through fare could be established.
Where a through fare was not shown in either the SPM or the DFM, then the station would either ring its managing station or call the DMO for the relevant add-on. Alternatively the famous phrase "rebook at XXXX" was used.
Clever booking clerks used to make a note of the fare in a book so that it could be used again.
It was the job of the junior clerk(s) to update this each year when the fares rose.
So that is how we used to work out a fare from insignificant station A to insignificant station D.
To establish
"availability", which by the way is a completely different thing to
"validity", the following criteria was used.
1 - The Selective Fares Manual (SFM)
The SPM would indicate the route where there were variations for example. A good case would be London to Cambridge which was priced differently from Kings Cross and Liverpool St.
2 - Reasonable Route
Is the passenger travelling by a reasonable route ?
Where a route was not shown the determination was by any reasonable. Unlike these days people were more sensible and would not try to argue ridiculous variations to the extent that people seem to try to do these days. It was also helped by the fact that the fares structure followed the train service, so for example Weston Super Mare was priced via Bath/Bristol Parkway, but NOT via Westbury.
Mostly this was an easy thing to establish as there were some standard easements, such as anything from London to north of Birmingham was available via Oxford, anything north of Manchester was available via Oxford and St Pancras/Derby/Sheffield/Leeds.
Carisle/Newcastle were interchangeable but not for break of journey on the off-route line.
Glasgow/Edinburgh and anything north thereof was available via the ECML/WCML/S&C and variations.
Taunton and westwards was available via Bristol, both routes, as well as via the WoE.
Exeter and west was available to Paddington/Waterloo.
I dare not comment on the Southern as it is a complete mystery to me, even today.
The general rule however was that if there was a train service or a group of services that took the passenger in the direction without being silly, then this was acceptable.
3 What if the Passenger is Off-route ?
Where the ticket was say via a specific route and the passenger was off-route then an excess fare was charged. This would be the difference between the original fare paid and the correct fare. This pretty much always only required the excess difference along the main route from intermediate station B to intermediate station C.
For example a passenger travelling from Crystal Palace to Cheadle Hulme via Derby would be off-route from London. The excess therefore would be calculated as follows.
Step 1
Original fare minus London to Cheadle Hulme.
Step 2
The sum of St Pancras to Derby, plus Derby to Stoke, plus Stoke to Cheadle Hulme
Step 3
Calculate the difference between step 1 and 2. This is the excess, subject to when using an Ordinary Return, half the difference only is payable unless the passenger is returning via Derby.
As a general rule this method worked for about 99% of cases.
What if the Route is completely Unreasonable ?
The passenger would generally be TI'd mainly to obtain details to go on record in case they came to notice again.
Hope this helps a little.
