Old railroad ties were usually treated with creosote. Especially with well-weathered ones, you don't need to worry about treating them like 'toxic waste', particularly, but the recommendation seems to be to put the pieces in ordinary trash that will end up in a landfill. Sad but true. You *don't* want to burn them, or compost them, or use them around your veggies..... If you google "old railroad ties" and "creosote", you'll be able to find out more.

I once lived in a house made of RR ties. I got a contractor to make some alterations and his people tried to cut one with a chain saw, hit a really gross metal spike and threw pieces of his saw all over the house.