Raised beds materials

No base, you want worms to get in and water and roots to get out.  Factor in the cost of lining the sides of the beds,  wet soil will rot even thick treated timber.  Any plastic sheet with some uv resistance will do.

just thinking about your treated railway sleepers what are they treated with?

Any "new" treated timber will be treated with tanolith E which is perfectly safe if they're reclaimed they could be treated with coal oil creosote which I personally wouldn't use. As mentioned before whatevever you use line with DPC, it's available in widths from 4 inch to around 18 inch so you don't get much waste, as for which timber to use for me depends on budget, what tools are availeable and how strong you are. An 8 foot sleeper is heavy, expensive and hard to cut up and join back together without the right gear but lasts a long time and for me looks better. Decking is cheaper, easier to work with but will last a fraction of the time. I've used both on my plot and both do a good job.