The welding process is in principle a regulated electric short circuit with the two rail ends functioning as D.C. poles. The high current produces an arc between the rail ends, the two rail cross-sections are heated to melting temperature due to the high input of current.
During the following upset stroke the rails are pressed together under high pressure causing the ends to coalesce. The upset metal is trimmed off immediately after the upset stroke.
The result is an accurate weld without filler metal with a very small heat-affected zone and a more or less consistent progression of the hardening process with a favorable crystalline structure