OP here. Thanks. I think for most I will book open return tickets and hope for the best on the return train but at least have the outbound train secure.

In the UK, I think it's much cheaper to book in advance.

It’s cheaper to book in advance but if you just want to wing it, you’ll be fine

Paris to Lyon 100% if you want the TGV. Fwiw a lot of it is sold out already. Everything else? No.

Anonymous wrote:(excluding Chunnel tickets) do train tickets from the main cities to other cities need to be booked in advance? Things like London to Cambridge, Oxford, Windsor, etc. or Paris to Orleans, Caen, Lyon, etc. Or can you just look up the train schedule, turn up to the station 20 minutes early and buy tickets? What about the return? I'm fine prebooking the outbound ticket, but I don't know what time exactly I'll want to return? But don't want to risk the train selling out.

Generally no, but you can sometimes get lower prices by booking in advance.

On the return, often (at least in the UK) you can buy an open return ticket.

I don't think any routes like that would usually sell out, but the one that I might worry about is Paris-Lyon, as that is more of a long haul trunk route. I'd be worried about say peak days/times during holidays, etc, but that's it.

(excluding Chunnel tickets) do train tickets from the main cities to other cities need to be booked in advance? Things like London to Cambridge, Oxford, Windsor, etc. or Paris to Orleans, Caen, Lyon, etc. Or can you just look up the train schedule, turn up to the station 20 minutes early and buy tickets? What about the return? I'm fine prebooking the outbound ticket, but I don't know what time exactly I'll want to return? But don't want to risk the train selling out.