Abstract: Mobile apparatus for, on site, welding of rail retaining clip studs to existing railroad rail plates which includes a railway car assembly having wheel and axle assemblies and propulsion means to move the railway car assembly along the railway tracks. The railway car assembly further includes two carriage assemblies each of which carries two welding gun assemblies which project downwardly upon each side of the rail plate. The carriage assembly includes carriage actuator means for moving the carriage assembly both longitudinally and transversely of the rail plate to position the welding gun assemblies. A position and sensing control means is provided which includes a rail plate sensor which first senses the position of a rail plate to stop the railway car assembly and properly position the welding gun assembies with respect to the rail plate longitudinally thereof.