The Hiawatha currently maxes out at about 62 mph, and makes a pair of stops between Milwaukee and Chicago (Sturtevant, just west of Racine, and Glenview). It's been historically considered one of the 3-4 most-successful Amtrak runs and is actually considered a commuter run by Amtrak. It does suffer the limitations that Amtrak puts on stops (no more than one per city).
The Metra commuter line that is being talked about uses a line that is much closer to the lake. It currently stops at Kenosha, and makes a lot more stops than the Amtrak train (same max speed limitations). There is no current station infrastructure north of Kenosha on the line that Metra would use. Morever, scaling the Metra price for Racine/Milwaukee would make it no cheaper than Amtrak for a much slower run.
One thing I was curious about: to what extent do any existing railway systems make use of an express/local system sharing one set of tracks (except for selected passing points) such that someone who wishes to travel 40 stops may travel 35 stops by express and then five by the local? In places where infrastructure could be adapted to support such a thing, it would seem that if it were well-coordinated it could significantly improve efficiency. I know Chicago has A and B trains, but what other such systems does anyone know about?