Growing up in Indianapolis, I certainly was familiar with the pork tenderloin sandwich, but never was a fan, probably due to inferior product being served in the metropolis. Maybe I should circle back and try some native product from other areas of the Hoosier State. The sandwich in question was generally referred to without reference to "pork" (just called a "tenderloin sandwich") and the ones I ate the most of, by far, were crappy imitation processed & frozen kind that were reheated in the school cafeteria. I think I may have had two total of the "real deal," both of which were eaten outside the city limits of Indianapolis -- but wasn't too enthralled with those, either, based mainly on a childhood aversion to pickles. (The archetypical dressing for a tenderloin, at least in Indiana, is yellow mustard and pickle slices.) Like so many Hoosier things, you really don't get the genuine article in Indianapolis, other than every August at the State Fair, at which time the urbanites on the North Side of Indianapolis are gently reminded what State they actually live in. If I happen to get to the State Fair this year I'll try to lay my hands on a tenderloin at the Pork Producers Pavillion -- which is really going to the source!
Here's a link to Roadfood's survey of Indiana restaurants, several of which feature the subject of this thread, with some pictures:
Hoosier Tenderloin Spots (and others)