It's an interesting one
I like simple clock face timetables (and I get annoyed at the "this will put out town on the map"/ civic pride arguments) but there are certainly some flows which warrant some kind of daily "extension"
Certainly, if a service can be run in "marginal time" then the economics look a lot better. As has been alluded to above, the Inverness - Kings Cross service came about because it was identified that one HST wasn't leaving Craigentinny until almost lunchtime (given that the cross border flows were a lot more "southbound in the morning, northbound in the afternoon" back then), and one HST wasn't used after arriving at Waverley at tea time - so you could run it from Inverness at eight in the morning so that it arrived in Edinburgh in time to take up the southbound path that would otherwise had been fed by an ECS move from Craigentinny - and then use an HST to provide a six o'clock service from Edinburgh to Inverness using a train that would have otherwise have headed back to Craigentinny instead.
This was all in the pre-Sectorisation days of British Rail, maybe such a use of stock would have been less likely once it became a case of "InterCity" encroaching onto "ScoRail" territory, who knows
This is based on the 1980s timetable, maybe nowadays it'd be useful to some if there were additional resources providing northbound ECML capacity in the morning - given that the fact that London to Edinburgh trains are generally based in Bounds Green/ Heaton/ Craigentinny, there's some long waits in the morning for northbound services, hence Northern having to use their Sprinters to provide a morning arrival into Newcastle from Darlington/ Durham - would it be better if LNER based an additional train at Neville Hill so that it provided additional capacity into Newcastle and allowed Northern to focus resources on the Durham Coast instead? Robbing Peter To Pay Paul, sure, but at the moment Northern are having to use their scarce resources to plug a gap in the LNER timetable because LNER are using their resources to save ScotRail from having to resource the Highland Main Line as much as they would have. At the moment there would be a gap of over half an hour in the northbound services from Durham to Newcastle (08:22 - 08:55) if it wasn't for Northern diverting a Middlesbrough service away from the Durham Coast line.
Anyhow, compare and contrast. The LNER service to Inverness uses units that would otherwise have been "spare" (albeit, I understand that they have a slightly complicated system where staff are put up in digs - I think that they the crews sit on the cushions of ScotRail services between Aberdeen and Inverness, since the evening arrival into the Highlands is too late for the staff to then be able to work the 07:55 southbound departure the following day?). The one XC service a day from Aberdeen to England needed the following:
05:17: Leaves Craigentinny (ECS)
06:32 Arrives (ECS) into Dundee
06:42 Starts in service from Dundee
07:55 Arrives into Aberdeen
08:20 Departs Aberdeen
10:54 Arrives into Edinburgh
11:06 Departs Edinburgh (to Plymouth)
.
18:08 Evening service arrives into Edinburgh (ex Plymouth)
18:13 Departs Edinburgh for Aberdeen
20:44 Arrives into Aberdeen
21:35 Departs Aberdeen
00:13 Arrives into Edinburgh
00:21 Departs Edinburgh (ECS)
00:29 Arrives into Craigentinny
...so that's running a Voyager for twelve hours a day just to serve Aberdeen - you can certainly reply that it wouldn't have been used for much else in those times, so it's "free" to use.
But there are costs to these kind of services. As well as the diesel etc, there are problems ensuring sufficient staff trained on the infrequent extensions. How many drivers/guards do you need to maintain route knowledge etc, given that staff may be sick, for the sake of one service a day? Do the XC staff only do a return from Edinburgh to Aberdeen and that's a day's work? i.e. two sets of staff required to run the two trips each day? But then is that cheaper than having a base in Aberdeen, where you'd have to give them accommodation and maybe pay ScotRail's staff at Ferryhill to provide some light maintenance on the unit that would stable there overnight?
Running one LNER train a day to Inverness or one XC train a day to Aberdeen (which, let's face it, it effectively is, there aren't
many passengers using the contra-flow journeys) can cost hundreds of thousands of pounds. And, whilst some will be tempted to blame privatisation, it'd be expensive to provide under any circumstances (since it'd be expensive/ complicated to have Ferryhill/ Inverness staff crewing it, given that it's not their standard traction, even if they do have the route knowledge)
Some other "one a day" services look quite simple/cheap to provide - e.g. if there's a Paddington diagram that starts/finishes at Temple Meads then it's not a huge cost to extend it to/from Weston Super Mare. Others though, they involve additional trains (e.g. one LNER unit north of Leeds at all times to provide a Harrogate train). If you can use the train to provide additional capacity into a large "provincial" city without much cost then great - e.g. LNER from Skipton providing extra capacity into Leeds in the morning peak - BR running an 87 from Ayr into Glasgow before it was needed for WCML duties - but when it starts requiring additional staff or additional trains then it becomes another story.
However there's always going to be a lot of love for quirky one-a-day services amongst the enthusiast community, and a disproportionate amount of anger about anything being cut even if it wasn't used by that many people
The LNER Aberdeen services replaces the ScotRail intercity service in the hours they run so by removing LNER services ScotRail would have to fill in the gaps to keep the hourly frequency.
That is a compelling argument, but I guess that two counterpoints would be:
1. What other services do ScotRail use with their HSTs, given that LNER and XC are providing services north of Edinburgh? (e.g. if LNER provide the teatime service from Edinburgh to the Highlands then does a ScotRail HST do a different job, or does it head to the depot prematurely?). When ScotRail operated fiftysomething 170s, you could have argued that a 158/170 "saved" from the Highland Main Line was providing additional capacity on something like the Fife Circle instead - but now ScotRail have "specialist" trains for their longer distance routes then they can't be used for other services quite so easily
2. When ScotRail were taking on fifty plus HST power cars, wast here any suggestion of them taking over these paths north of Edinburgh?
The ECML does seem to have a hell of a lot of extra trains going off grid, and arguably wasting capacity. Certainly when compared with GWR which has very few "add ons" overall, although I do recall the train that once ran Abergavenny to London via Hereford!
I could be wrong here, it's going back a few years, but was this when the service was running ECS from Phillips Marsh, so was running ECS through Abergavenny anyway? In which case I'm all for that kind of marginal use of stock (but it'd be quite an additional cost if it was having to a Hereford - Abergavenny - Hereford round trip twice a day)
Its a hard one. These services are popular and important, especially beginning/end of service extensions in marginal time. But, if by removing them you can improve woeful service provision on the trunk, as is the case with XC, then perhaps they should be culled.
Agreed - hard to provide a "hard and fast rule" but some are certainly a lot harder to justify than others!