I have very, very vague memories of travelling on some Network Southeast stock in the mid 1990s from (I assume) Poole to Wareham to visit the Swanage Railway. I must have only been maybe 3 years old at a push. Still have a photograph of tiny me on the stock with my ticket, which is clearly Mk. 1 or old Mk. 2 outline stock with the big windows - I thought the route was already in the hands of 442s at that point, but maybe it was a drag or a summer loco-hauled service?
Other than that, no memories pre-privatisation. Have to go to a trip down to London on a HST in 1997 for my first real memories - I remember being bought a crap kid's mealbox from the buffet car and playing a boardgame on one of the tables, which it was etched into for passengers, but my only actual memory of the visit to London itself is visiting the McDonalds just down the road from St. Pancras for some reason.
It just about pre-dates a trip to Sheffield to ride the trams on another HST, with similar consequences. Since I was obsessed with trains and anything that ran on rails, it was an easy and cheap way for Dad to take me somewhere and keep me quiet without having to do anything particularly taxing - I'd just have my face glued to the window and he could take a nap. I wouldn't actually be surprised if that trip to McDonalds was literally all we actually did in London.
Coincidentally, that Sheffield trip was most memorable for the train actually missing its stop at Loughborough, slamming on the emergency brake and having to turn back, which caused considerable delay! I thought this was a far-fetched story I had imagined as a child, but my father has confirmed that this actually happened! It can't have been that far-fetched, since a
similar thing happened at the same station in 2014.
Most of those early memories revolve around my excitement at spotting, and then riding on, a "Virgin Train", which again was a HST, to Didcot in this case to visit the museum there. For some reason most of my early on-board memories seem to centre on the buffet car provisions - in that case, being baffled and amazed at the existence of Virgin Cola.
You can see where my love for the HSTs stems from - every one of my formative rail trips which I can actually remember took place on one.