"Induction welding" ...
For a really technical answer, I'll have to leave this to someone with better technical knowledge than me (as you guys know, I'm in "customer service" not "technical support" ... and for very, very good reasons), but let me give this a little stab (after consulting some of our tech guys ... like my pun?).
Induction welding is the processing of passing an electric current through the two metal surfaces (in this case, the bolster and the liner), thus heating them up to the point of fusing them together.
I hope that works for you (and that I'm right).