Fish plates are fixed in rail tracks to:

Fish plates are essential components in railway track construction. Their primary function is to join two rails together. These metal plates are bolted to the web of the rails at their ends, ensuring a continuous and stable track. They maintain the alignment of the rails, prevent vertical and lateral movement, and allow for thermal expansion and contraction. Without fish plates, the individual rail sections would not be securely connected, leading to track instability and potential derailments. The other options describe different functions or parts of a train or track system.