Flash Butt Welding
Flash butt welding is the preferred process which involves an automated track-laying machine running a strong electrical current through the touching ends of two unjoined pieces of rail.
The ends become white hot due to electrical resistance and are then pressed together forming a strong weld. Thermite welding is a manual process requiring a reaction crucible and form to contain the molten iron.
Thermite-bonded joints are also seen as less reliable and more prone to fracture or break.