Which section(s)? At Newton Heath & Moston Metrolink station (formerly Dean Lane railway station) the tram line through the location has effectively been singled so as to keep it completely separate from the parallel heavy rail line (to the nearby waste depot site just to the East).
From Shaw & Crompton to the junction at Rochdale had been singled by BR in ?1980. It's now double as far as the Newbold tram stop.
The revisions to the Salisbury to Exeter single line have been very considerable since the 1967 singling to allow the current service, which is way ahead of what was provided then. Someone must have a complete list somewhere of what was done. Not only reinstatement of some double line, and additional passing loops, but also Intermediate Block Signals which allow trains to follow more closely one another on the single line.
The rail overbridge on the a357 Templecombe High Street is only single track. Roadwise its only traffic light controlled alternative working with a height of 13 ft. So any modification would require a complete rebuild of the bridge. I think the bridge being singled when the spur line to the SDR route closed in 1966/7 allowed the route of the spur line to be used as a new road to what is now the Thalys factory. You have to balance this against the benefit of extending the double track. Templecombe only achieved just over 100k passengers in 2019 so its one of the least used stations on the line.
The revisions to the Salisbury to Exeter single line have been very considerable since the 1967 singling to allow the current service, which is way ahead of what was provided then. Someone must have a complete list somewhere of what was done. Not only reinstatement of some double line, and additional passing loops, but also Intermediate Block Signals which allow trains to follow more closely one another on the single line.
Only those parts detailed by @DrHoo, there are still some single sections.
Lugton/Stewarton is several miles of double track rather than just a longer loop
TB (Tokenless block) bidirectional working was retained on the up line while the reinstated down line became AB (Absolute Block). All the LSWR/Southern equipment had been stripped from Yeovil Junction SB but luckily it hadn't been demolished so was able to be reequipped with Western Region equipment (that part of the line was WR managed at the time).
The St Pinnock / Largin single line section was introduced to reduce loadings on the viaducts in 1964. It's under a mile long. The second former up track on the Penzance approaches become an independent access line between the station and Long Rock depot. The single line there is just over one and a quarter miles long. The Royal Albert Bridge single section is under a mile long. Little chance of any of these being redoubled (Royal Albert never was double of course!)
I call it 'half a Cambridge'. Split platforms with turnouts or scissors part way along are nothing new at large stations. They're particularly common in Holland, an early proponent of short frequent train services, to make best use of very long platforms. Penryn is a small barebones implementation of the same concept which, most importantly, saved the cost of an accessible footbridge crossing to the other side.
Old maps suggest there were actually two completely separate 2 track bridges, with space for a road junction between them. I do think as you say the factory access is over the old SDR west to north curve, but removing that pair doesn’t seem to have automatically made it necessary to single the main line bridge. When looking at Streetview I suspect a straight extension of the present Throop Road would have run right alongside the main line to a T junction, on the west side of the High St there’s quite a long abutment.
It had always been a bay line, and the down equivalent was only a siding.
Which is a slightly curious one as it seems to be of limited use - like the southern loops at Llandeilo and Llandovery, there are no passenger trains timetabled to pass there. Train crew route knowledge limits mean that regular services can only pass each other at Llandrindod or Llanwrtyd.
Before Knighton, I recall a claim that the 32 miles between Llandrindod and Craven Arms was the longest single line section in the UK at the time. I guess it could also be useful, along with the other more rarely used passing places, for service recovery after disruption. I don't fully understand how route knowledge could affect where trains can pass. Is it related to crew changes en route.
You are correct; the side road between the Templecombe bridges, Throop Road, was the way to the isolated platform on the S&D line, quite a distance from the main station, to the end used for odd services to save them having to do the shunt up to the main station. The last train of the day from Bournemouth used to terminate there, which as I think Throop Road had no street lighting or pavement must have been a challenge for some after evening closing time! I think the remaining single rail bridge is new, since the singling. It must be the most bridge-bashed of all modern bridges, far too low for an A-road, always smashed on both sides. The Thales industrial site, a high-tech military research place, completely out of character for a small Somerset village, is on the site of the old S&D loco shed, and seems to stretch a good way up the old alignment towards Evercreech. Apparently the business started off in the old S&D shed building there.
The trains pass each other, but the crews don't - they swap units and head back in the direction they came from. Crews from the south only sign as far north as Llandrindod, while those from the north only sign as far south as Llanwrtyd. So the crossing point couldn't be switched to Knighton, as the crew coming up from the south wouldn't have route knowledge for the Llandrindod to Knighton section.
Interesting, I wasn't aware of that.
Trains pass at Llandeilo at around 7.05am each day
They also pass at Knighton at around 7am each weekday
So they do, I'd forgotten those early morning part-route services. And the northern equivalent from Llandrindod to Crewe does appear to cross a southbound train at Knighton, which answers my question.
The cost of maintenance doesn't AIUI reduce in anything like that proportion to the number of trains, unfortunately, and there can be a significant one-off saving if renewals are due. Singling can also provide opportunities to increase speed or loading gauge by slewing track to the centre of the formation (though that isn't cheap!).
While not on the scale of the Uckfield line singling, quite a few sections of the Strathclyde suburban network were partially or completely singled in the late 80s, Dalreoch-Balloch, Craigendoran-Helensburgh Central and Westerton-Milngavie (the sections in/near Bearsden and Hillfoot stations act as passing loops of sorts, with Milngavie itself being a two-platform terminus) being the ones that spring most readily to mind. IIRC, all of these were in tandem with the closure of individual signal boxes and the opening of a new signalling centre at Yoker. Part of me thinks that if this work were to be carried out now, singling any of these wouldn't even be a consideration regardless of the cost savings.
Only Westerton - Milngavie was singled (in part) during the Yoker resignalling scheme, in 1990.
Cheers, always had it in my head for some reason that all 3 had occured around the same period. Didn't realize (or had completely forgotten) that Dalreoch - Balloch was singled as early as 1973 though. Likely confusing it with the closure of Balloch Central/Pier and the crossing on Balloch Road.
A close relative of singling lines is single lead junctions, and these were installed at a number of locations on the Strathclyde network as well. I never quite got the claimed maintenance benefit of them, how two powered points and a diamond crossing were more expensive than four powered points. Plus there's the capacity issue on the now singled connection, making scheduled parallel moves impossible, let alone the safety issues of a single track section without any of the safeguards, tokens or whatever, you get on a full single line. Nor was flank protection possible at the junction any more.
It’s even more complicated than that…..Gilmour Street - Arkleston remained four track post-electrification in the 60s, was rationalised to 2 tracks in the 80s and put back to four recently.
Not south of Kilmarnock. There was a singling scheme in the 80s which got as far as cables being laid and relay rooms being built before cancellation.
Ardrossan-Largs was singled in the 80s to facilitate electrification, as was Hamilton-Ross Junction in the 70s.
