Apologies for the naivety here, but I'm surprised that I couldn't find a single flexible weekend working job opening when I did a quick browse of railway jobs. Especially given the current strike situation, I would have thought it would be attractive to workers and TOCs alike to offer weekend-only positions for some roles which I'm sure would be attractive both to those who are employed during the week and want additional income on an ad-hoc basis, as well as to those who want an initial opening into a longer-term railway career.
I guess my rationale is that gets incredibly frustrating as a passenger to see 'cancelled due to a shortage of train crew' on the departure board, knowing that I would be more than willing to work one or two weekend days a month to fill untaken slots. I appreciate there will be an upfront training (and recruitment) cost, but I'd have thought it would be beneficial in the long-run, especially if it helps reduce weekend rotas for full time employees (for those that would prefer not to have them).
I am a complete outsider to the careers side of the railways so don't be too critical if I've misunderstood something.