Amtrak is resuming rail service on the popular Borealis line Thursday, using bi-level railcars on the route between St. Paul and Chicago.

Trains from the Windy City to the Twin Cities will begin rolling Thursday night with service in both directions expected to be back on track by Friday morning, the railroad said.

Passengers with tickets on Train 1340 from St. Paul to Chicago on Thursday will be bused to their destination, Amtrak said.

"Service by Borealis Trains 1333 and 1340 will be restored without bus substitutions," the railroad said.

Amtrak uses single-level Horizon railcars on Borealis and four other lines across the country, including the Hiawatha, which runs between Chicago and Milwaukee.

On Wednesday, the railroad discovered corrosion on "multiple" Horizon cars and sidelined them. Passengers were bused to their destinations, Amtrak said.

Richard Tsong-Taatarii, Star Tribune
Amtrak CEO Stephen Gardner welcomes passengers Candra Thomas and her mother Billie, 87, as Amtrak’s Borealis daily service to Chicago begins from St. Paul’s Union Depot in May 2024.

Borealis service began in May 2024 and uses single-level Horizon railcars built by Bombardier between 1988 and 1990. A total of 70 cars — 61 coaches and nine food-service cars — were affected, according to Trains.com.

Bi-level railcars, known as Superliners, will be shifted onto Borealis and some runs of the Hiawatha line.

The bi-level railcars do not have business class. Passengers with business class tickets on Borealis will be given a refund to cover the difference between business class and coach, the railroad said.

Amtrak discovered corrosion on Borealis' railcars this week during a routine inspection in compliance with federal requirements. More intense inspections turned up "additional areas of concern" and Amtrak decided to remove the cars from service, spokesman Marc Magliari said Wednesday.

The line, which provides one trip between St. Paul and Chicago in each direction daily, has been extremely popular. In its first 22 weeks, more than 100,000 passengers hopped on, the railroad said in October.

Service on the Empire Builder, which runs from Chicago to the West Coast cities of Seattle and Portland, was not affected. The Empire Builder stops at Union Depot in St. Paul