Without knowing the specifics (and subject to the disclaimer that I'm not a lawyer or anything like that)
From what i can see (I'm not a railway worker either but considering it) the franchises moving in to public ownership is a change of ownership of the existing company, so that doesn't even count as a change of employer, so pay / terms + conditions don't change. (nor does 'continuous service' for things like redundancy pay entitlement.)
When staff are transferred from one employer to another, there is law (it's called TUPE if you want to research) which means employees keep existing terms and conditions and pay rates and continuous service. (the law is slightly fuzzy round the edges especially when it comes to detail, though, and it doesn't cover pension schemes.)
Employers can seek to change terms and conditions, irrespective of TUPE transfer, but can't just impose it with no consultation - and in a unionised environment, not without a fight. In extreme cases, some employers will do the 'fire and rehire' thing, but not sure that would get through on the railways, and there are suggestions that the current government intends to make this illegal.
There is an urban myth that TUPE protection lasts for a fixed amount of time, but this isn't law. Although the sooner after a transfer that new employer seeks to change conditions, the easier it is to argue that it's a direct result of the transfer.
Others will have direct experience of what's happened when rail franchises have changed hands. It wouldn't surprise me if there is a mix of staff on current franchise holder's terms + conditions, some on previous franchise holder's terms + conditions, and a shrinking number of staff still on BR terms + conditions. (I've dealt with this in other lines of work.)
Some employers will try and encourage TUPE staff to go on to new employers terms and conditions, sometimes with a one-off payment to 'buy out' some conditions. Again on the railways, I would expect the unions to try and 'level up' rather than 'level down' - different grades of staff will be in a stronger position than others.