What woods were used for railroad ties in 1915?

Answer 1

they were made of any tree that was near buy then they coved them in tar. but if they had them shipped in they were usually of a variety of hardwoods, and are often heavily creosoted or, less often, treated with other preservatives

Answer 2

To the best of my knowledge, oak has always been the choice for railroad ties. I'm sure that whatever trees were convenient were used as the railways were extended across the US.