Surely they should accept a birth certificate ... So many organisations just assume that everyone drives or has a passport.
Remember that this is for an online application - birth certificates are accepted when presented in-person at a station.
The one number that is always correct for you is your NI number, as as far as I know it's issued when you're born.
They are actually issued through an automatic process when a parent claims Child Benefit. If no such claim is made, or the person was born outside the UK, the NI number is issued manually upon that person's applying for one. There are also a few circumstances in which an adult will not have an NI number, such as if they're not permitted to work or claim public benefits.
As just about all gov. departments ask for your NI number as your third form of id
It's important to note that Railcards are issued not by a government agency but by ATOC Limited (trading as Rail Delivery Group), which is a commercial organisation. They will not have access to the HMRC records necessary to validate an NI number and associate it with the holder's date of birth.
Does the driving licence have to a new Photocard one or will my original green paper licence last issued in 1984 do?
I can find nothing on their website saying that it will work with a paper licence, but neither can I find anything saying that it won't - so either "yes, it will work" or "no, because the designers never considered that it would need to work with paper licences".
(as an aside I'm curious as to how many are still validly circulating, since any change of address or other renewal now triggers replacement with a photocard)
If the choice was between paying £34 for a driving licence and £70 for a railcard now, or paying £90+inflation for railcards over the next two years...
I got myself a provisional licence as soon as I could after I moved here as it was the easiest way of getting my photograph and address side-by-side on the same official document. That's one of the joys of living in a country without a single standard national identity document: driver's licences inevitably become the de-facto vaguely-standard national identity document