I can't say I remember one particular journey - more like 'snippets' of a number of journeys - the chronology of which I don't know because I was too young.
I recall 4-CORS (Nelsons) because of the distinctive cabs, and feeling that the interiors where solid, quite stylish and relatively luxurious compared to other, more modern trains.
I also have memories traveling in trains hauled by steam engines. Here the experience was more to do with seeing the train arrive and depart rather than the travelling: The steam and smoke, the hissing and puffing, the palpable change in the air, the frisson of pressurised power in the boiler - it made the hairs on the back of my neck stand up
As a passenger the steam heating also seemed to have impressed itself upon my young mind - although I've never been able to put my finger on exactly why.
As an older schoolboy the old SUBS where a favourite over 4-CEPS because of the single compartments. At stations the three or four of us would stand firmly pressed against the door and windows to make the compartment look like it was totally overcrowded so no one else would get in