Fishplate

In rail terminology, a fishplate, splice bar or joint bar is a metal bar that is bolted to the ends of two rails to join them together in a track. The name is derived from fish, a wooden bar with a curved profile used to strengthen a ship's mast. The top and bottom edges are tapered inwards so the device wedges itself between the top and bottom of the rail when it is bolted into place. In rail transport modelling, a fishplate is often a small copper or nickel silver plate that slips onto both rails to provide the functions of maintaining alignment and electrical continuity.

The device was invented by William Bridges Adams in May 1842, because of his dissatisfaction with the scarf joints and other systems of joining rails then in use. He noted that to form the scarf joint the rail was halved in thickness at its ends, where the stress was greatest. It was first deployed on the Eastern Counties Railway in 1844, but only as a wedge between the adjoining rails. Adams and Robert Richardson patented the invention in 1847, but in 1849 James Samuel, the engineer of the ECR developed fishplates that could be bolted to the rails.

The moving blades of a set of points can be connected to the stock rails by looser than normal fishplates. This is called a heeled switch. Alternatively, the blade and stock rail can be a one piece heel-less switch, with a flexible thinned section to create the moving heel.

When railway lines are equipped with track circuits, or where the line is electrified for electric traction, the electrical connection provided by fishplates is poor and unreliable and has to be supplemented by bonding wire fixed to the two rails either side of the joint by spot welding or other means.

Even though fishplates strengthen the weak points represented by rail joints, improvements can still be made. For example, the joints can be welded together using the thermite welding process. In 1967, at Hither Green on the Southern Region of British Railways, a major disaster occurred when a rail fractured at its fishplate joint. Welded Rail installation was sped up due to this error, with strict procedures on concrete and wooden Sleepers.

...
Wikipedia