Why is sound produced when the train moves over the track

The rails (permanent way) are of standard length, and wheels of the coaches (under carriage) are fitted at standard places underneath the coaches. When the coaches are on the move the wheels transit from one rail to the next one and because of a small gap between the rails the sound is produced. In view of the standard length of the rails and the placement of the wheels under the coaches there is the rhythm.

When the train is running at high speed the interval between the rhythms is less while the interval is long when the train is running at slow speed.

To reduce the rhythm (sound) Indian Railways are welding the rails with alumino thermit process without use of fish plates. However the welding is done for a set of five or six rails and at that point the noise is inevitable but yet rhythmic. Passengers travelling on trunk routes would experience smooth running of the train.

Indian Railways are also experimenting with the rails that have their ends cut at 45 degrees angles. In this when the coach is on the move from one rail to the other (the next one) there is no sound since the wheel is already on the next rail, before leaving the earlier one. In this the fish plates have to be used instead of welding the joints.