I'm no expert on this subject by any means, but I happen to volunteer at a museum where we have steam trains running. We build our tracks to look traditional, so we use wooden sleepers and no ballast. Most of our sleepers are donated from the commercial railroad companies, typically they are old stock but we also receive used ones occasionally. In my part of the world wooden sleepers aren't common anymore, so it's getting harder to find usable ones. This is a concern for us, as apaearantly there aren't any suppliers left in our part of the world for new ones. At our museum they typically last for about 15 years, mainly because we place our sleepers directly on the soil (no balast). The tar/oils will eventually dry out and the wood will just rot/decompose naturally. Wooden sleepers are considered chemical waste in my part of the world, though I do believe we are allowed to let them decompose fully as biomatter, which goes quite quick if in contact with moist soil. Though we typically dispose our used sleepers at a specialized waste facility, I'm not sure how they process it there.
Oh, and in case you are wondering: no, they don't burn, so we can't use them as firewoord for our steam engines ;-)
Appearantly in the USA, at least as of 2008, around 90% of all track was still using wood [1]. I didn't expect that. For most of the world we have used concrete sleepers for a long time already. Plastic sleepers are also common nowadays, which are typically made from recycled materials.
[0] https://en.wikipedia.org/wiki/Creosote [1] https://en.wikipedia.org/wiki/Railroad_tie