People on here really don't like terminating anything short of where it could end up (e.g. the thread about extending Glasgow - Manchester to Birmingham, but then you can't terminate things at busy old New Street so you might as well extend it through to Bristol, but then why not go the whole hog and make it Penzance... actually, it could start further north of Glasgow, maybe Inverness... no, I know, Thurso, let's start it at Thurso...) - they hate the idea of buffer stops - even if it means knocking down half of central Bradford!
Yet, when it comes to bus services, a lot of people on here love the idea of terminating bus routes at some suburban roundabout so that people are forced to change to a tram (it's apparently fine for buses to terminate in the middle of nowhere and people be forced to change, but bad if rail passengers have to change anywhere...
I share some of the frustrations with branches - they can be inefficient uses of resources, they can require non-standard stock (e.g. a diesel branch in an electrified region, the 139s for Stourbridge, the hassle in getting trains between a branch with a PVR of one and a distant depot) and therefore something to try to avoid/simplify - but that said, I can see why it might be better to have a short unit doing the shuttle than either running twelve coach trains from the main line to the branch or running short trains from the branch onto the main line... tricky
The three problems with a shuttle (as I see it) are:
1. Does it terminate at a useful destination in its own right, or at somewhere that's only really a junction?
2. Could/ Should/ Would the frequency be any better if it was a through service/ would it be better to have a reduced frequency on the branch if they were through trains?
3. Is there scope for through trains on the main line?
In the case of Cornwall, I'd suggest that Par is a pretty rubbish place to terminate Newquay services (it's not much of a destination in its own right, compared to the Falmouth branch which terminates in the regional centre of Truro) and that there's nothing stopping an increased frequency on the Newquay branch with just one unit (whereas the Falmouth branch is pretty much at capacity) and there's nothing stopping through trains from Newquay to Plymouth on a more regular basis
For the Thames Valley branches, they look fine as they are, pretty much operating at line capacity, you wouldn't want to run short DMUs through to Paddington and there aren't any spare paths at Paddington so any Marlow/ Henley/ Windsor services would have to be at the expense of an existing Reading - Paddington service, so I wouldn't change them
South east London I know less about. I can see why the Bromley shuttle annoys a number of enthusiasts, it's short, it feels incomplete, especially with various other "incomplete" bits of line nearby (the Hayes branch takes up four paths per hour at busy London termini, Tramlink and the DLR aren't far away, you could waste a lot of crayon on coming up with something that ties all of these frustrating "stubs" together. Instinctively I can see it's "troublesome" to use heavy rail paths from central London on fairly short routes like Hayes or the Sutton/Wimbledon loop since there's an opportunity cost to these paths (and they could be used for longer distance/ longer trains) but it's not a hard and fast rule
But what would it achieve if Bromley - Grove Park was a through tram from Croydon or DLR to Canary Wharf, if the main demand would still be for connections into London Bridge etc? If the branch is running as frequently as it can (or wouldn't see any significant increase in frequency if it was trams/ DLR) then what's the advantage for most passengers? You could have a Bromley North - Bromley South extension but how many links does that provide (that aren't already available from the different ways of getting from the Orpington and Swanley lines to the Bromley South/ Grove Park lines)? Bromley North already seems close enough to the main shopping area, but would you be putting an intermediate station between there and Bromley South if you had a tram/DLR linking both sides of Bromley (the two existing stations are less than 1km apart)? I dunno, it feels like a lot of effort (and money) for the sake of very few genuine benefits, other than appealing to members of the BufferStopPhobic community
Agreed - it'd be much better with a dedicated unit, rather than hamstrung with an awkward timetable because the railway is still catering to the days of wealth Victorian Mill owners travelling from West Yorkshire to "Bradford-On-Sea"