There's the cost of the train combined with the associated cost of the maintainence to go with it.
If you've reduced the number of trains but are still paying for fundamentally the same big shed and staff to maintain the trains that are left then the saving isn't as significant.
A Saturday standard hour service (with 12 cars) probably still needs 90-95% or so of the total fleet requirement for a weekday peak.
This is an interesting subject.
Just had a look for the GN route, I've picked this fairly randomly simply as it's a route I'm most familiar with.
MF - 717 - 21x diagrams, 4x stable between peaks (3x in major depot, 1x in carriage sidings).
SO - 717 - 10x diagrams, but not a fair comparison as the service is half the MF off-peak service.
MF - 387 - 25 diagrams - 6x stable between peaks (all in major depot)
SO - 387 - 26 diagrams - 7x stable between peaks (all in major depot)
MF - 365 - 17 diagrams - 17x stable between peaks (all in major depot)
SO - 365 - 0 diagrams (all stabled in carriage sidings)
MF - 700/1 - 23 diagrams
SO - 700/1 - 23 diagrams
MF - 700/0 - 11 diagrams - 3x stable between peaks (2x in major depot, 1x in carriage sidings)
SO - 700/0 - 7 diagrams (with not quite a full off-peak service).
Because of the difference between the 717 MF and SO service it's hard to make a direct comparison, so working on the basis that 4x 717s stable between the peaks we can say that an all-day off-peak 717 service could be provided with 17x diagrams.
So if we work on the basis that the MF timetable has 97x diagrams, an SO timetable would require 73x diagrams, with the difference being 24x diagrams, which is roughly a 20-25% reduction in diagrams. As you no doubt know the Saturday service does in this instance provide a very small number of additional "with flow" extras, and some strengthening from 8 cars to 12 cars on busy 387 services at tourist times.
It is of course worth adding that the 365s are maintained at a time when Hornsey Depot would otherwise be relatively empty.
Going through and modifying all this on a number of vehicles basis to account for the variations in lengths, it comes out as follows:
MF - 717 - 126
SO - 717 - 102
MF - 387 - 100
SO - 387 - 104
MF - 365 - 68
SO - 365 - 0
MF - 700/1 - 276
SO - 700/1 - 276
MF - 700/0 - 88
SO - 700/0 - 56
Total - MF - 658 vehicles
Total - SO - 538 vehicles
So that makes the SO service require 82% of the number of vehicles required for the full MF service. Naturally the moment any additional services are required at busier Saturday times then this percentage would start increasing quite rapidly, even if they're only out for one trip.
Please feel free to double check my figures!
