Reading the above I realise that being in your eighties has the compensation of being able to remember with nostalgia many of the above. A further one that springs to mind is the climb from Liverpool (Riverside/Docks?) up through various tunnels to Edge Hill (having arrived in 1961 on the Empress of Canada). The trip out to Canada involved a through journey from Evercreech Junction to Liverpool Lime Street. Also several trips on the Normandy Express from Waterloo to Southampton Terminus whilst at university at Caen.

 
The old Midland route into Bristol via Mangotsfield 1962 Worcester to Weston

Hull to Hedon on the Withernsea branch

Goose Hill Junction to Normanton

Bradford Exchange

 
I did both of the above, the Manchester to Brighton one I used a lot. That also had the bonus for a few weekends because of engineering work between Reading and Oxford. The train still ran but went via the West Coast Mainline after leaving Olympia, it didn’t stop at Reading, Oxford, Banbury, Leamington Spa. The first stop was Birmingham International which thanks to the lack of stops and higher speed we arrived at, before the train should have reached Banbury. I have vauge memories of waiting to join the WCML possibly near Willesden Junction with the tannoy reminding passengers where we are not stopping.

I also did what became the parliamentary service between Wandsworth Road and Kensington Olympia. I believe that went over track that is not in regular passenger use anymore, hence the parliamentary service.

 

Unusually among the "no longer available", Blackfriars to Holborn Viaduct was replaced by an alternative when it closed with few (if any) train journey opportunities lost.   I often looked at the derelict tracks down to the widened lines, after through freights has cease, and thought "what a waste ..."

 
I have happy memories of branch lines at the far ends of journeys from London for family holidays in the late 1950s-early 1960s.

• Waterloo direct to Woolacombe and Mortehoe (made famous by Flanders & Swann), and separately on the same holiday the big climb from Ilfracombe to Woolacombe and Mortehoe.
• Sidmouth to Exmouth changing at Tipton St Johns (I didn't suffer motion sickness on a train whereas a bus was more risky).
• Waterloo to Swanage.
• Ryde to Shanklin.

And, like many members, Blackfriars-Holborn Viaduct.

 
One that springs to mind was in 1965, travelling from Renfrew Airport on the late plane(£5 single!) for a weekend in Crewkerne.
Came to land at Heathrow in thick fog, approaching the runway the engines of the Vanguard fired up at full blast as the landing was aborted, 30 minutes later we landed at Manchester and were taken to Central Station where they had laid on a special train hauled by a Peak for my only journey over the Midland Main Line.

Another was my first railtour, again in 1965. This was run by the St Andrews University Railway Society and used J39s to haul the train over the Fife coast lines, ending at Dundee West on its last day before closure.

Then there were the A4s doing the Glasgow Aberdeens via the old road no longer in use.

 
Back in 1971, I stayed with a spotter friend in Lincoln. My trips to and from Derby landed me at Lincoln St Marks.

 
Winter '73, and on a late evening - or was it the overnight service that left Kings Cross for Leeds about 11pm, and was much diverted along the way, after other entertainment the train ran through Normanton having used that now abandoned line south of the station. The station at Normanton gave a very convincing impression of time-travel in that judging from the scene through the window we seemed to have slipped back to around 1952.

It may have been one of the journeys that took me over the Lincoln avoiding line but looking at a railway map I don't know if that would have been possible, but in any case, not a lot of time was spent on the usual route to Leeds. Thinking about it, another of these involved passing through Cambridge, through March and then via the March - Spalding line which I actually recall as being very steady progress and remarkebly dark night-time countryside through the window.

Mark

 
Have been on Folkestone Harbour branch (including far platform on a rail tour)
Sifin branch in Derbyshire
Direct curve at Old Oak towards Kensington Olympia
East Putney to Wandsworth flyover line

Once got very early first train from Kingston to Waterloo which included Wimbledon staff halt in the destination list and uses interesting crossovers

 
Other comments on this thread remind me:
+ Dover Marine
+ Paddington to Greenford direct

Overall - hundreds and perhaps as many as a thousand journeys in and out of Holborn Viaduct, nothing else such regular / frequent use.

 

Ditto, I think.

 
Shortest aside ever:- 
The crossover from the Down to Up Relief at Twyford on a Paddington - Henley train.

 

Indeed! That was my latest, but yet which I forgot - Friars Junction to Greenford, taken on one of the last Chiltern parliamentaries into Paddington.

Also now I think about it, I was on the curve round from Old Oak Common to North Pole Junction that the old Brighton Cross-countries used to take. Short but sweet.

Oh, and does the original connection between HS1 and the Southern third-rail network (when Waterloo was international) count?

 
For me I think it's just Dover Marine, the Folkestone Harbour branch (on the UK portion of the Citalian Express no less) Holborn Viaduct, the Lincoln avoiding line, also Gunnislake (the station across the road, but perhaps that's wishful thinking). Bradford Exchange of course, and I recall zero publicity that it was to close - the replacement as first built was squalid. Also, Forster Square - the new station's just about off the site of the old, and for good measure shares with St Ives in that the new station's on the previous fish dock. Ah, and Ashton Gate from the harbour direction. Oh, and Gogarth, but it was a station and not an entire line. Ah, and Lincoln St Marks - which *was* on its own line.

Come to think of it, that last bit of the GWR line from Birmingham into Paddington at Old Oak Common as well. HSTs from Bath to London were sent to Oxford and then that way to Paddington. The train crawled the last few miles. That's now gone, hasn't it?

New lines include Tweedbank, and the Airdrie and Bathgate (though mostly asleep for that...)

Mark

 
Two that come to mind on the same University railway society tour: Preston to the Red Scar sidings on the old Longridge Branch over the M6 - the site of the line is now a slender footbridge. Plus Poulton-le-Fylde to Burn Naze on the old Fleetwood Branch.

As a youngster on a family trip to western Scotland we travelled into the old station in Fort William, with its lovely three road starting signals and the track that ran out along the quay beyond the station. Now truncated about 3/4 mile back towards Mallaig Junction.

Even younger - the Douglas to Peel line on the Isle of Man. Can that count as GB?

 
I was fortunate to travel between Cheltenham and Stratford-on-Avon on a railtour in '75.

Of course part of that route now is occupied by the G&WSR.

 
The "local" of my youth:
Havant, Langston, North Hayling to Hayling Island

Moat Lane Junction to Llanidloes, having changed from the Cambrian Coast Express

 

Following in from the thread on "three railways I wish I had experienced", I've written myself a similar list - but this time not trimmed - of those I did use before their demise as part of the national network

Firsby to Lincoln
Shanklin to Ventnor
Taunton to Minehead
Haltwhistle to Alston
Wareham to Swanage
Alton to Winchester
Uckfield to Lewes
Tunbridge Wells Central to Eridge
Paignton to Kingswear
Blackfriars to Holborn Viaduct
Broad Street to Dalston Junction
New Holland [Town] to New Holland Pier
Craigendoran Pier to main line
Balloch Central to Balloch Pier
Paisley Canal to Kilmacolm
Glasgow High Street to Bridgeton Central
Spalding to Boston
March to Spalding
Aberystwyth to Devils Bridge
Selby (direct) to York
Bedford St Johns (Old station)
Darnall to Penistone
Sandhills to Liverpool Exchange

Some are very vague memories - but some are strong even if a long time ago ...