Why is a small gap left at the joint between two rails?

Friends, have you notice the small gap left at the joint between two rails? There is a big reason for doing this work. You know that all metals expands on heating and during summer season temperature of rail tracks increases due to heat generated by friction of moving trains, which is also responsible for expansion of tracks. If there were no gap at joints, the rail tracks would have expand randomly in summer season, which can be reason for train accident. While if there is a gap, the expansion will take place in the gap only and no chance of disturbance in tracks will occur.