For openers, it's important to remember that it was a case of Pennsy-come-lately in the Motor City. I'm attaching a link to a
New York Times article from 1922 which discusses the PRR's decision to penetrate what surely seemed like the most promising market in the nation at the time.
http://query.nytimes.com/mem/archive-fr ... 946395D6CF
The PRR's plan was to use another road's trackage in the already-congested Toledo-Detroit corridor, and the Pere Marquette's Saginaw-Toledo main apparently offered a better deal than did the Detroit & Toledo Shore Line, a jointly-operated venture by Grand Trunk and Toledo, St. Louis & Western (also called the Clover Leaf -- later a Nickel Plate/N&W property). Passengers were handled at the Fort Street Union Depot, built thirty years prior to the Pennsy's entry, but PRR apparently planned on building and operating some segments of trackage entirely on its own. How much of this actually came about, and how much of a role might have been played by terminal roads like the Union Belt isn't clear.
The New York-Detroit
Red Arrow was, of course, the focal point of PRR's Detroit passenger service until around 1961. I've encountered the term
Iron City Express before, but in reference to service to Pittsburgh from the East; it seems possible the Pennsy decided to extend that run once the opportunity was there. And a 1946
Official Guide in my library also makes reference to a Chicago-Detroit sevice in collusion with the Wabash. I'm not sure what other Pennsy passenger runs entered the city from the south and east; my interests run more toward freight service, and I do know that the PRR "unoffically" carded a direct Enola-Delray time freight, westbound only, symbolled ED-3; the predominately-eastbound flow of freight traffic in the US would lead me to believe that, like the Pennsy's fleet of westbounds symbolled "LCL", this service was used at least as much to return empties as to solicit the high-paying, but demanding merchandise traffic.
Hopefully, some other members here can fill in the blanks on what I view as one of the most-neglected portions of the PRR empire.