This is likely happening because the shoe is making contact with the center rail on the switch. First, check that the uncoupler shoes on the passenger car trucks are in good order (no chips or rivets hanging out, and that the shoe is mounted and sprung properly).
Once the trucks look good, go take a look at your switch and with power, slowly move the offending car(s) over it. You will likely observe the point of contact. I would wager it is the rivet (actually stud) that holds the center rail.
There are many ways I have heard of people resolve this from careful use of ball-peen hammer, dremel, file, sanding or replacing the rivet (actually, it's a stud, postwar part O22-14). However, usually people forget that the bottom of that stud as a screw that may work loose due to vibrations, so I would try that first:
See the exploded diagram here: http://www.olsenstoy.com/cd/swt/stc022e.pdf
