If the offer is still conditional (pending things like references / medical and so on) then I'd say it's definitely not time to resign from current job, only when you've got it on paper and a start date has been agreed is it time for that. With some jobs, even once they have gone through the references etc it can be a while before a start date comes along (if it's the sort of role where they do a set number of training courses each year, for example.)
And in the current situation, even then, it would be worth having a talk with (new) employer about how they are going to handle you starting / training in whatever phase of lockdown it is then (bearing in mind that whatever lockdown rules are in place at the time you get the offer may not be the same by your starting date.)
I had a couple of conditional offers in early spring this year (one railway, one other) - one was withdrawn entirely, the other got deferred and in the end I did not feel confident starting a new job and learning new systems etc entirely on 'work from home' basis so declined rather than risk getting laid off after a few weeks if it didn't work.
My understanding (subject to the disclaimer that I am not a lawyer) is that once you have got something 'on paper' and signed it, then both parties can still give notice before employment actually starts, and generally speaking, there are few rights to claim unfair dismissal or redundancy pay if you've been employed less than 2 years. Many years ago a new employer went bust a few days before I was due to start there, and there was not a lot I could do about it other than get to the job centre.
Ultimately (once you do get a firm offer) then it's going to be a case of balancing how keen you are on the potential new job, how secure your current job is, what the chances are of getting a similar opportunity on the railways when / if things are back to normal if you turn this down now, what your chances are of getting back in to your current line of work (or at least of getting another job) if the new job doesn't last, and whether your finances and circumstances are such that you can afford to take a risk. I'm not expecting you to go in to detail on any of this in public, of course.
