Why is there a small gap between two railway lines?

The gaps you are referring to the small gaps that are left between section of the rail that form each side of the parallel railways for over which railway engines, coaches, trains and trams run.

These gaps which are of the order of a few millimeters, are provided to allow room for the rails to expand the rise in temperature due to the atmospheric temperature as well as the friction caused by running of train. All materials expand when heated and contract when cooled. the extent of expansion and contraction is quite low to be noticed by people or make much difference in most situations. However, since each section of the rail is very long, the expansion due to heating is significant, and unless space is provided for expansion of rails, the rails will bend to accommodate the increased length. This will make the railway unfit for running of trains.