Formatting somewhat improved. But putting figures in based on a milage solution shows up the real problems of an inversion of the usual purchasing rules in the current system.
"The more you buy, the lower cost it is per unit" is a pretty standard thing when you go into the supermarket - compare the costs of a large, medium and small jar of coffee / bag of potatoes / box of cereal. And yet on train fares we currently have an inversion of this - the short journey is often cheaper per mile than the long one, even on the same tracks, leading to the perverse practice of split ticketing.
My default fare test for a 100 mile journey such as Melksham to London gives me:
Projected anytime fare £59.90 single and £115.80 return
Projected off peak fare £35.94 single and £69.48 return
Which HMG will object to because a normal anytime return is £215.20 and a superoffpeak is £68.10
But if I change only the 100 miles to 25 miles - example Melksham to Swindon - it gives me
Projected anytime fare £17.00 single and £31.40 return
Projected off peak fare £10.20 single and £18.84 return
Which the passenger will object too because a normal anytime return is £12.50 and off peak is £9.20
This is without any of that bias at all that I have talked about station-to-station but even if that were pushed to the extreme for Swindon - Didcot it would not fix this inversion issue. Changing the fixed price for going a train to a negative value could help and would make very local journeys (Westbury to Dilton Marsh?) free of charge or in theory end up with the railway paying you (and that would be the end of fare evasion on local West Wilts services
)