Hmm, well, looking at a January 1970 Official Guide of the Railways, which should be able to give a good idea of the fallen flags that served Detroit in the 1950s, below are a number of the companies that served Detroit:
Pennsylvania Railroad
New York Central
Norfolk & Western
Detroit, Toledo & Ironton
Chesapeake & Ohio
Baltimore & Ohio (had connections via trackage rights, so I'm not sure if they actually served any plants)
Wabash
Pierre Marquette (although, by the 1950s they were a subsidiary of the C&O, of course)
Both of the Canadian roads, Canadian National and Canadian Pacific
The Nickel Plate may have also had trackage rights including the Ann Arbor and Grand Trunk Western, although again, I'm not sure if they actually served any of the plants in Detroit.
These are not all of the lines, I realize, but I believe that the PRR, NYC, and C&O (not counting the Canadian roads) for the most part had the market cornered in Detroit.