A commuter route needs a good service to get people to/from the hub to fit the working day and a basic service during the day.
How would you define "basic", ellendune?
I know I'm not Ellendune, but I've been looking at "minimum sensible service level" recently - TransWilts specifically, but also comparing that to further afield.
As I see it, there are three different mentalities we passengers use when travelling by train / bus / ferry / plane:
a) Travel by train if there happens to be a conveninent service
b) Decide when you wish to travel, and then find a service close to that time
c) Turn up and go
(a) is the only option with a service that operates infrequently - as on the TransWilts. I got back to Chippenham at 21:57 on Friday evening, and (having missed the 19:01 to Melksham) decided not to wait for the 15:38 the following day (the next train!) but rather I planned to use the bus (last bus failed to turn up, ended up shelling out for a taxi, but that's another story).
For option (b) to work reasonably well, you need a service who's frequency is such that you can adjust your journey time by about the same amount of time as the journey takes to find a convenient service. For example - if I wanted to travel from Swindon to Westbury at 17:30, a 40 minute journey, I would be happy to plan on getting a train as early as 16:50 or as late as 18:10. There are "working day" questions here where people with fixed appointments will only be able to travel earlier in the morning, and only later in the evening. And further questions about people not wanting to wait the extreme time at both ends of the day.
(1)For option (c), I would suggest every 10 minutes (urban), 20 minutes (suburban) , 30 minutes (intercity) are reasonable. I'll cheerfully turn up at Paddington for a Chippenham service and get on the next train that's available (or would if I wasn't paying personally - I'll usually delay to offpeak because I can afford to waste time occasionally!)
My view - option (a) is only viable for train services designed to meet specific passenger flows along a corridor rather than a more general travel flow. Examples here might be works trains - looking to past services which operated to stations such as IBM, British Steel Redcar and Sinfin. And you may include pure commuter flows and special in there too, such as some of the South Eastern services into Cannon Street, and perhaps in our own area the through trains from the Thames Valley branches into Paddington, but in those latter cases these are supplementary to other trains serving the same stations, and you need to be certain that you cater for people who may have to stay late at work / have different hours occasionally. If I worked from 8 till 6:30 in Swindon and lives in Trowbridge, I might use the present TransWilts service - but only because I have an alternative on that 1 day per week when I start or end significantly earlier or later.
Option (b) - if you work it out for the TransWilts - is a train every 90 minutes in each direction. It's woolly round the edges - there's no immediate switch from a service at level "a" to a service at level "b", and in any case there's a wide variety of journeys - only a small percentage end to end so full time - on this line. So I'll accept a service "at least once every 2 hours" as a
minimum sensible service level at which traffic will naturally flourish and grow, bearing in mind that you need a corridor on which there are travel needs, which we certainly have. Service at 06:30, 08:50, 14:25, 17:45 and 22:10 - the old Wessex service from Swindon - fall far short of the tip-over point to option (b).
Where there is substantial traffic on a flow, an increase above the (b) level will make a big difference - see
(1) above. So you find that a quieter branch line works with a single train at basic level, and a busier one gains substantially from having a loop in the middle with trains passing each other there.