Mainland....So you weren't on the Isle of Wight?
To be clear, do you mean "oldest train" as in "inside oldest carriages" or as in "behind oldest locomotive"?
I think it's not so old that the answer can't be 'both'
Manchester Piccadilly? (having taken the trans-Pennine train from Sheffield via Grindleford)
Nice try but even older than a Pacer
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The oldest trains through Sheffield would be the High Speed Trains (HSTs) run by what is now East Midlands Railway and Cross Country Trains. The newest in that part of the world are LNER's new Azumas. Cross Country and LNER meet up at Doncaster.
It wasn't up north
The answer is Brighton.
Guess you travelled on Volk's Electric Railway?