Why Do People Put Their Ears to Railroad Tracks to Detect Approaching Trains?

[m.l.]

Homework Statement

People put their ears to a railroad track to get an early warning of an approaching train. why did this work.

Homework Equations

written response.

The Attempt at a Solution

Sound waves can travel a long way along an undisturbed metal rod?

 

[Kurdt]

The rod has to be disturbed for any sound to be carried along it. Apart from traveling a long way in the rail, what other property does sound have in the metal that is greater than in air that would be an advantage?