In the US, trains are seen as ancient cowboy technology. Trying to build more train infrastructure is surely an uphill battle.
So in order to get a few hundred cars off the road, Amtrak would have to build a holding track (carbon cost), add more rolling stock to the route, burn more diesel, and a residential neighborhood full of children would have had diesel trains idling for hours a day.
The total economic and environmental value of the program was negative by most measures.
1. Amtrak ridership will barely grow, at most by a few hundred riders per day. If you're this bearish on trains then naturally you'll think nobody wants to take the train and you don't think Amtrak should build more trains. You're coming into this assuming that nobody wants to ride the train.
2. Building dedicated track between Chicago and Milwaukee would be cost prohibitive for Amtrak or otherwise infeasible. The majority of the cost for building track comes from acquiring ROW (so purchasing the land), grade separations, and utility relocations needed for laying the track. Chicago to Milwaukee in particular is one of the cases where these costs are probably _lowest_ as much of the ROW is cheap to acquire (outside of the direct Chicagoland area), the land is fairly flat so grade separations are rarely needed and utility relocations are cheap to build.
3. The opportunity cost in carbon for added vehicles on the road would somehow be less than the opportunity cost for rolling stock to idle. Even if freight rail continues to refuse to electrify in perpetuity, the added carbon from private vehicle emissions will quickly dwarf extra carbon emissions from idling freight cars.
I think you're coming at this from a "trains are stupid, here's why" perspective rather than an unbiased cost and carbon perspective. These arguments change in areas where ROW acquisition is expensive or grade separations and utility relocations are difficult, but Chicago to Milwaukee has very few of these problems. In particular the problem in this part of the US is that while rail works to move from city-to-city, most Midwestern cities (other than Chicago) have no actual transit to speak of. Once the Amtrak drops you into Milwaukee, if there's nowhere to go via train, then you're stuck, in which case you may as well drive the whole way. That goes back to the fact that rail/transit in the US has been historically and systematically underfunded, particularly so in the Midwest where Detroit had a lot of negotiation (read: money) leverage in the golden years for American auto companies.
Especially if we are talking about installing high speed lines anyways, doesn't make sense to turn a freight line into a high speed line and then share it (other than the fact that you wouldn't need to eminent domain as much).
The bathrooms are less claustrophobic though.
If you think that building rail viaducts and rail grade separations are carbon expensive, wait till you learn about the carbon used for roadway viaducts and roadway grade separations.
You can argue that rails usually run on much more consistent of a grade than roadways but that's only to decrease the amount of acceleration that's required by train cars and to increase top speeds. Freight rail is a special case because freight companies try to avoid as much wear-and-tear on their train cars as feasible within their delivery windows, unlike passenger rail which necessarily cares more about speed and less about wear-and-tear.
Vastly fewer, per passenger mile, then passenger cars, comparable to busses, less than scheduled airline flights.