ST. PAUL — Adding a second round-trip train between St. Paul and Chicago could prove to be popular, a new Amtrak study said.

The study predicted ridership on a second daily train could exceed 150,000 passengers a year, but Minnesota and Wisconsin would have to subsidize about $6.6 million in operating shortfalls annually. It also said the necessary infrastructure improvements could range from $142 million if the route ends at St. Paul’s Union Depot to as high as $257 million if stops are also added at Target Field in downtown Minneapolis and St. Cloud.

The Minnesota and Wisconsin transportation departments as well as La Crosse County in Wisconsin asked Amtrak to conduct the study.

Amtrak’s only train now serving the Twin Cities is the once-daily and often late Empire Builder from Chicago via St. Paul to the Pacific Northwest. About 100,000 people ride it annually between St. Paul and Chicago. Eastbound trains leave St. Paul in the early morning while westbound trains leave late at night. Rail network congestion from oil train traffic in North Dakota and Minnesota has contributed to the problem.

The proposal for a second Amtrak train is independent of discussions about starting high-speed rail service between St. Paul and Chicago.

The study assumes the train would follow the same route as the current one, with a top speed of 79 miles per hour, with the addition of a Milwaukee Airport Rail Station stop.