FWIW, I believe that the original trolley case came from Foot and was an attempt to do a version of the airline pilot case without uncertainty.  (Rails add certainty to trolley cases.)  So on that understanding of the case, the person switching is in some sense already driving and hence already on track to kill someone.

Subsequent discussions seemed often to turn the person into a bystander by a switch.  Such a bystander is not on track to kill someone unless she switches the trolley. 

I've always thought that is a relevant difference in that the first sort of scenario is kill one vs. kill five.  The second sort is kill one vs. don't prevent deaths of five.

Recently Judy Thomson has espoused a similar view in response to some input from one of her former grad students.  See: JUDITH JARVIS THOMSON, "Turning the Trolley," Philosophy&Public Affairs, 36 NO: 4 (2008): 359-374.

This was why I gave "need more info" or one of the "other" options as my answer to this case.