It would make a cool museum space I think. You could showcase Philadelphia's industrial past. Bring in some antique locomotives. The light coming in from the ceiling vents and the size of the tunnel would be pretty cool. Now all that would require probably hundreds of millions to seal the tunnel from the elements and make it into a usable space, so it won't happen and probably rightfully so. Every cool idea shouldn't be done necessarily. It was built as a transit tunnel, it makes the most sense to continue to use it as such.
"imagination and memory are but one thing, which for diverse considerations hath diverse names" - Thomas Hobbes