Amtrak passengers at Chicago Union Station on Wednesday morning found their plans had gone slightly off track. Instead of navigating the downtown terminal's subterranean passageways, they lined up on the crowded sidewalks of Jackson Boulevard to shuffle onto shuttle buses that are running in the tra ins' place — for the foreseeable future.Amtrak said it began busing passengers between Chicago and St. Paul, Minnesota, and Milwaukee after discovering "corrosion" on several of its passenger cars, raising the prospect of long-term service disruptions.The passenger rail service said it is running buses in place of trains on its Borealis route, which runs a daily train to and from Chicago and the Minnesota capital. Shuttle buses have also replaced half of Amtrak's six daily Hiawatha service trains to and from Milwaukee. Passengers load onto a bus heading to Milwaukee outside Union Station on Wednesday.Anthony Vazquez/Sun-Times Amtrak removed several of its single-level Horizon railcars from service after discovering corrosion, the company said in a statement Wednesday. Amtrak did not say how many cars were removed from service or how long the bus substitutions would last.The Hiawatha line had an average of 1,800 daily riders last year, according to Amtrak figures. The Borealis route, which Amtrak began last May, averaged around 420 passengers a day.Dan Hun, 23, was standing alone on the sidewalk of Jackson Boulevard, staring at his phone in confusion about which shuttle bus he should take to return to Milwaukee."I feel a little bit weird," he said, angling his phone to a reporter to show there was no information about where the bus pickup would be.Minutes later, one bus from Milwaukee arrived with dozens of passengers who had received texts and emails earlier that morning about the switch. Amtrak passengers were directed to shuttle buses on Wednesday at Chicago Union Station.Anthony Vazquez/Sun-Times Brian Schuster did not have any complaints after the hour and a half ride to Chicago."It was great," the 56-year-old said.He learned about the switch to a bus by a text from Amtrak around 2 a.m. He said that was "plenty" of time in advance.Tricia Francis was less impressed after stepping off the bus."I wasn't too happy about it," said Francis, a 26-year-old nurse who traveled to celebrate her mother's birthday in Chicago. "I'm used to big buses, but the Amtrak is just a smoother ride."Don Carr, 69, stood in a line of several dozen people waiting to board a shuttle bus to Milwaukee. He takes the Hiawatha line about five times a year to visit his daughter.He said Amtrak communicated the change well and he did not mind the switch to a bus."To go to Milwaukee — not a big deal. It's a short ride," he said.Two other routes — the Downeaster line in New England and Amtrak Cascades on the West Coast — also have been affected by the shortage of passenger cars. ...read more read less