Beginning Wednesday, D Street will be closed between Yellowstone Highway and Eastern Avenue.
Idaho Falls city engineer Kent Fugal says it's a 14-15-month project.
Construction was supposed to start Monday, but Fugal said the contractor behind the project is moving materials to be ready by Wednesday. He says the underpass has been in Idaho Falls for about 100 years.
The underpass is being replaced for two reasons: to create more room for traffic underneath by expanding the narrow lanes and to update the underpass.
Fugal says the new underpass will have "standard width lanes, shoulders, two lanes westbound so we can have a dedicated left turn lane to turn onto Yellowstone to go southbound."
The entire underpass will be demolished. To keep train traffic moving the city's Public Works Department will build a temporarily rail line.
The underpass will be closed 24 hours a day and seven days a week. Elm and Ash streets, along with South Boulevard are recommended detours.