The original headquarters was right across the street from the current Amtrak station from 1920 to about 2000. It's a very significant building in that inventions such as the artificial heart, automatic transmission, and air conditioning were invented or perfected there and also the war effort or "arsenal of democracy" was heavily concentrated there. Lots of history making moments between the HQ building and the engineering building across the street, both adjacent to the current station.
GM moved downtown when both GM and Detroit were in trouble and the Renn Cen had high vacancy. It's ironic as that building was a Henry Ford II project around 1978 to revitalize downtown.
I've never been a big fan of the Pontiac dogleg. From what I understand, the reasons were two-fold. (1) to provide an endpoint on GTW after the station was shifted to New Center and away from MCS; (2) to tap into the viable and populous northern burbs.
That is a crummy reason. By that logic, why doesn't the Lincoln Service start at Naperville, go to CUS, down to STL, then out to Clayton? Why doesn't the Wolverine have a western terminus in Geneva or Aurora? Chicago, New York, LA, Washington, Boston et al... riders get on their corridor trains downtown.
We're told that Detroit has far less riders than Pontiac or Troy, but close those stations and make said riders come to Detroit or Ann Arbor and the math changes. Fun fact, it's far faster to drive to Ann Arbor then ride to Chicago than it is to ride Pontiac-Detroit-Chicago.
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