I have my engineering curiosity hat on and thinking about the manufacturing problem that arises when electric spot welding connectors to the rails. This is conjecture.
The contact and rail materials are dissimilar metals and I think that the contact material has a lower melting point. So the amount of current in the spot welding process may be based upon the lower melting point of the contact which is borderline for a good weld to the rail.
Perhaps a better idea is to solder the contacts to the rails in lieu of spot welding them.
