Great Western Coffee Shop

This ran yesterday hauled by 37521 + 37688 for Locomotive Services according to RTT.
Can anybody provide more detail?
According to Tracksy it is still in Jersey Sidings this morning.
It seems to spend today doing a couple of trips to Weymouth and back before setting off for Crewe via Eastleigh tomorrow.

Listed as "private charter", operated by Locomotive Services, and it (or at least the same traction) ran Crewe/Llandudno/Blaenau Ffestiniog/Llandudno/Crewe on 19th-22nd May and then Crewe/Llandrindod Wells/Weymouth/Crewe on 22nd-24th. But as to why ...

Jersey Sidings presumably being previously connected to the Weymouth Quay tramway (see separate discussion) for boat trains to the Channel Islands. A nice nod to the past.

The Jersey sidings (on the left) used to be used for the Weymouth Wizard HST to lay over before returning at the end of the day.

(http://www.mbob.co.uk/rforum/jersey.jpg)

Well, "connected with"? Only in the sense that both connect to the main line. And while you'd think the sidings are a small remnant of the railway yard, in fact that was all the other (east) side of the line. Where the current sidings are wasn't even really land until about fifty years ago, but the "sea" wall of Radipole Lake. It was the so-called development plan post-war that reclaimed the edges of the lake for roads and stuff, though the reeds for the birds were later on preferred to dredging for boats.

Why Jersey, then? Perhaps an earlier name for a siding somewhere else?

The Jersey sidings were added in 1938/9and are the only ones now available. The Jubilee sidings were added in two groups, the first in 1935, King George V’s silver jubilee, and the second at the same time as the Jersey sidings. The land used for the Jubilee sidings is now a commercial estate.
The footbridge was a good vantage point, especially for the engine shed, now a housing estate. Incidentally, while going around the shed one evening we were invited onto the footplate of 34015, Exmouth, by the cleaner and then allowed to drive it back onto the turntable.

The map comes from TRACK LAYOUT DIAGRAMS OF THE GWR AND BR WR SECTION 17 WEYMOUTH LINE By RA Cooke

A single Jubilee Siding remains available for use, but it’s a very far cry from what used to be there, as shown in the track diagram you posted.

The relatively recent addition of sleepers across the London ends of the two remaining Jersey Sidings prevents their use for run-round moves these days. I’m unsure whether Network Rail would make special arrangements for them to be removed if a train operator did require to use this facility.

As an aside, going back to the OP, the headcode for this train was 1Z65, 155R being the obfuscated version displayed on RealTimeTrains, Traksy, etc.

Looks like it left  today at 07:26 https://www.realtimetrains.co.uk/service/gb-nr:U68274/2022-05-24/detailed#allox_id=0 (https://www.realtimetrains.co.uk/service/gb-nr:U68274/2022-05-24/detailed#allox_id=0)

Further poking about in "The Backwater" shows that, as a tidal estuary with restricted outlet to the sea and lots of people living around it, its natural tendency was to become a foetid swamp. As the Wey was navigable, it already had a weir in Weymouth, next to the sewage works. But it was still seen as a problem, and in 1919 money was voted for a new dam at the Westham Road embankment and a new bridge. Then in 1923 work to erect Radipole Park Drive started, and it opened in 1927. That brought with it the strip of land back to the railway, where the new sidings were built, as well as loads of tennis courts etc. It was said that GWR would build a subway at "the crossing", presumably of the tramway at Westham Road, but I don't think they ever did.

There's no sign of a master plan, and arguing about how much new land to create and what to do with it was still going on after WW2. Birds didn't feature much, until the RSPB had enough money from a big appeal to buy the swannery in 1976. Radipole Park Drive, in a way that has become familiar, looked like a good way to remove traffic from the centre of Weymouth but didn't connect to the existing main road at its northern end. It came out in residential streets, and the first bit was unmade and had not even been adopted by the council before 1929.