Question

In earlier times, people used to listen for the arrival of a distant train by putting their ear to the rail

Answer 1

It's because in earlier times technology was not advanced...
They have to do their work by own...

Answer 2

Most of what you would hear were vibrations caused by the wheels moving over the joints. Metal is an efficient conductor of sound and so the sound could propogate further in the rail than it would in air (and get around corners as well).