Explain why, wooden (or concrete) sleepers are kept below the railway line.

There are many reasons. Some of them are

To keep the lines equidistant from each other.

To reduce unwanted vibrations in the track.

The sleepers have a large surface area compared to the lines; this larger surface area decreases the pressure due to the weight of the train on the track. If only the rails were used without slippers; they being thinner would extent greater pressure over the bed of stone and move deeper in to the ground