I have plenty of sleepers but they are that old that they dont
'noticably' leak anything into the soil although I wouldnt recommend
sitting on them in best clothes esp. in warm weather as they can get a
bit sticky.

Line the inside with pond liner (using a staple gun - a strong one if
very hard wood sleepers) to keep the soil from going through the gaps
(there not totally square after 100+ years).

p.s. get a friend or two to help you lift them (carefully).

Nick