Amtrak’s route from Chicago to St. Louis would seem an ideal place for the U.S. to adopt high-speed rail such as in Europe and Asia, where passenger trains can race along at 200 miles an hour. The stretch in Illinois is a straight shot across mostly flat terrain.

In fact, a fast-rail project is under way in Illinois. Yet the trains will top out at 110 mph, shaving just an hour from what is now a 5½-hour train trip.

After it’s finished, at a cost of about $2 billion, the state figures the share of people who travel between the two cities by rail could rise just a few percentage points.

Behind such modest gains, for hundreds of millions of dollars spent, lie some of the reasons high-speed train travel remains an elusive goal in the U.S.

The challenges faced by Illinois, among them limited federal funding and people’s ingrained travel habits, mirror those faced by other states that would like to upgrade the passenger rail network.