The main function of a fish plate is to join the two rails together. It is a metal plate that is placed between two rails and is secured in place with bolts. The fish plate helps to distribute the weight of the train evenly across the rails and prevents them from spreading apart. It also helps to keep the rails in alignment.
Option B is incorrect because the fish plate does not join the rails with the sleeper. The sleeper is a wooden or concrete block that supports the rail. The fish plate is placed between the rails and the sleeper, but it does not attach to the sleeper.
Option C is incorrect because the fish plate does not allow the rail to expand and contract freely. The rail is allowed to expand and contract by the use of expansion joints. Expansion joints are placed at regular intervals along the track and allow the rail to move up and down without damaging the track.
Option D is incorrect because the fish plate does serve a purpose. It is an important part of the railway track and helps to keep the rails in good condition.