In old times, before there were train signals, people who worked on the railroad would often put their ear down to the train track to listen for an oncoming train.

why would this result in hearing the train further off than just listening for the train through the air?

people did not do this because sound travels faster through less dense material, like air, rather than more dense materiel, like train tracks.

sound waves travel faster in more dense material, and the more dense the material the faster the sound is.

sound travels slower in air because there are often trees and other items blocking the sound.

sound travels slower, but louder through solid material because it is more dense, so the train could be heard better this way.