Happy memories there, Phil.

I went to prep school at Stoodleigh, near Tiverton. Always went back to school by car, but rail home. Usually Tiverton to ESD, then Taunton to Castle Cary, on a couple of occasions via and changing at Yeovil Pen Mill. And on a couple of occasions when there was extensive flooding at Stoke Cannon, then Tiverton, Dulverton to Taunton and Cary. Hey ho.

 
Back in 1967 (I was 10 years old), my parents decided to have a family day out on the Hoe at Plymouth. We were a family of 6 and a car that would only take 5, so dad offered both my brother (who was 14) and I the opportunity of travelling down from Exeter (Pinhoe station had closed by then) to Plymouth by train. Of course, we were both train nerds and the opportunity of passing both Newton Abbot and Laira sheds was too good an opportunity to miss!

We go to the Exeter St David's ticket office and are asked 'did we want to go via Newton Abbot or Okehampton?'. Of course, you can guess the answer.  With the benefit of hindsight, the biggest mistake of my rail enthusiast life! I never did get to travel over the SR route to Plymouth!

My father was an avid golfer and a member of Tiverton Golf Club. He used to drop me off at Tiverton Junction to do some spotting whilst he played golf. I wish I could have travelled on the Tivvy Bumper!

Later on in life, I went to Bath University. I wish I could have had a trip on the S&D   

So, in summary......
1.  SR route to Plymouth;
2.  Tivvy Bumper.
3.  S&D
 

 
Two halves of what should have been one line but never were

   Shoreham by Sea to Christ's Hospital
   Christ's Hospital to Guildford

Now cycled most years as the Downs Link

   Chichester to Midhurst

Partially cycled most years as the Centurion Way

I didn't include Brighton to Kemp Town because at this range I am not sure whether I actually travelled on it or not. The memory gets mixed up with my first fag (Perfectos) was I on a train to Lewes and just saw the Kemp Town viaduct.

The mists of time!

 
I am amazed at the hugely wide variety of lines selected here.  I wondered as I asked the question whether I should run a poll.  I "chickened" on that, not knowing what options I would choose ... and suspecting that the most votes would be for "something else".

Our regrets at missing out on these lines are absolutely not suggestions or justifications of any potential re-opening campaigns, though it is interesting to think about a scenario where things might have been different.

 
My English grandparent's back garden abutted against the old Nottingham Suburban Railway Line from Daybrook Junction through Sherwood and into Nottingham (Victoria I think).  It closed before I was born but I have a picture of me being carried by dad in my grandparents back garden watching the last train on the main line going past Daybrook Station.
As children, my brother and myself had many happy hours on the deserted railway lines as it was a quick hop over the back garden fence.  I've often thought what it would have been like actually travelling on the lines and can't help thinking how useful they would now have been to the city's transport system.

 
Didcot, Newbury and Southampton.
Swindon, Andover to Romsey
Southampton Terminus -  I may have done this as I was born before it closed and I did live in Southampton.

 
Renfrew Branch - walked past it most days when at college in Paisley. My planned trip on it was changed when we were offered a footplate ride behind a black five to Elderslie Junction and back.

Killin Branch - spent hours at Stirling watching the trains to there but never caught one.


Castleman’s Corkscrew - Brockenhurst to Poole via Wimborne. I only ever did the Poole to Wimborne section.

 
Hayling Billy

Isle of Wight

Severn Bridge

 
Somerset & Dorset, of course (I live on top of it!)
Withered Arm
Port Road (to Stranraer)
Waverley Route (in theory I just could have done)
Callander and Oban

 
I'm rather parochial on my three:

North Cornwall Railway, to Padstow, where we used to holiday;

Plymouth to Yealmpton Branch, through Plymstock and Elburton, where I used to live;

The Primrose Line, to Kingsbridge, near where I met my wife.

Obviously, there's also the Portishead Line - but that is being rectified, so I can hopefully travel on it at some time before I fall off my perch.

CfN. 

 
The Somerset and Dorset Joint would have been high on my list.

 
Andover to Swindon, with a diversion to Tidworth  (I did Andover-Marlborough, but it was in the dark.)
Lambourn Valley Line
Sidmouth Junction - Sidmouth

(And if I had a time machine, the Larkhill Military Railway between 1917 and 1922)

 
Didcot to Winchester, Oxford to Cambridge and Carlisle to Edinburgh on the Waverley line.

 
My three would be:
Exe valley, Dulverton (Brushford) to Exeter.
Somerset and Dorset Bath to Bournemouth.
Three Cocks Junction to Moat Lane.

 
Newquay to Truro via Perranporth.

Lostwithiel to Par via Fowey

Plymouth to Princetown

 
In was 10 in 1964, 15 by 1969 ... travelling but not really aware of the depth of what was going on around me.  So many missed opportunities - whether they would have been practical I doubt.  Another thread (on the Bridport Branch) lead me to thinking which would I have most like to have travelled ... and I have come up with three - just three to avoid the list getting too depressingly long
* "Beyond Barnstaple"
* Ruabon to Morfa Mawddach
* Bridport Branch

What are your three regrets?