I was involved in a job for a retirement home, where they wanted a
raised bed for wheelchair access.

Railway sleepers were the choice.

However 'real' railway sleepers are prohibited for this use, due to
the seepage of tar etc. The seepage last for years and years, not just
2. The tar residue can burn the skin, as can the old type of creoste
preservatives.

I now its a bit late, but you can buy new untreated sleepers.