A couple of places with 2 stations built by separate companies I think could be merged.
St Albans City and Abbey, Windsor and Eton Central + Riverside, Bradford Interchange and Forester Square are some to name a few.
Plans to link Interchange and Forster Square have existed more or less ever since they came into existence, but nothing ever happened, and then just when it was looking as if they might do it at last some idiot sold the land which had miraculously remained more or less clear up until then...
It's an even bigger shame that it isn't possible to have one big station in place of Manchester Piccadilly and Victoria. Or indeed, that the (admittedly rather vague and nebulous) ideas concerning one mega-terminus in central London for all the different lines never happened.
And to keep them away from ladies of the night. The university has a lot to answer for but thankfully the much needed development of this quarter of the city will go a long way to balance things.
And the university admits women now
Windermere: Was in a fairly convenient location until BR sold the land to Booths supermarket, so everyone has to walk the extra distance to get to the station.
Agreed. Would have been even better if the railway had gone all the way to the lake.
And on the same lines, if instead of going over Shap the line had been built up the Kent valley and tunneled under the fells at the head to come out down Mardale, so Kendal at least would have been on the main line.
Euston Square - would obviously have been more sensibly sited closer to Euston.
Or at the very least had the exit, or another exit, at the closer end.
Or a subway. It is said that there is some other underground structure in the way, but surely it is possible to go underneath it.
Bank - putting the Waterloo and City Line platforms a bit nearer the other platforms would've been nice

(Would have required extending the tracks a little).
... put 'em at a lower level and have the GN&C at a lower level too (so as to be able to go under the stuff that is in the way), and the W&C to mainline gauge with a mainline connection at Waterloo, and link 'em up.