"J. Chisholm" wrote
Railcards say 'valid until:'
Is it reasonable to assume they can be used on the date stated?

Yes, I believe so. I bought my railcard on the 11th April 2008 and it says
'Valid until 10 APR 09', implying 'up to and including'.

Incidentally, the print quality on my railcard is so poor that the number
cannot now be read. On Tuesday, this prevented the LU ticket office at
King's Cross St. Pancras from enabling my Oyster card to benefit from the
lower daily cap, as loading the railcard number is apparently a key part of
the process.