Question

When we put our ear on a railway line,we can hear the sound of approaching train even when the train is far off?

Answer 1

Since metals have high density so it is obvious that the sound of train coming will be heard far before than we will hear from the other source ie. Air.
Speed of sound depends upon the density of source it is travelling from..

Answer 2

Because the track is made of metals and sounds travel faster in solids than air.