Came across this while making reservations to take bikes to St. Louis on Amtrak next month.

Between May 1 - 9 and May 16 - 24, Amtrak passengers will be transferred to motorcoaches for the section between between Springfield and St. Louis.  Though not stated in the article below, Amtrak customer service says the motorcoaches will not transport bikes.

Illinois High Speed Rail Work Resumes
April 16 - 25, May 1 - 10 and May 16 - 25, 2012
Starting April 16, the Illinois Department of Transportation (Illinois DOT) and Union Pacific Railroad will again be upgrading Illinois' signature high-speed route, Chicago-St. Louis, for future high-speed trains. This phase of the project will be performed during the three periods shown above and will affect the Lincoln Service and Texas Eagle. Details on the next phase will be provided as soon as they become available.

The temporary service changes described below will allow Union Pacific crews to improve infrastructure to enable the Lincoln Service trains to travel at speeds up to 110 mph (177 kph), an increase from the current maximum of 79 mph (127 kph). Illinois DOT and Amtrak are planning to preview higher speed trains in September 2012 from Dwight to Pontiac.

Lincoln Service
On April 16 - 24, May 1 - 9 and May 16 - 24, Lincoln Service trains will operate between Chicago and Springfield only. Alternate transportation will be provided between Springfield and St. Louis, as described below. Due to limited availability, advance reservations are required.

Southbound Trains 301, 303, 305 and 307
On April 16 - 24, May 1 - 9 and May 16 - 24, all passengers destined for, or making connections in, St. Louis will be provided express motorcoach service upon the train's arrival in Springfield. Passengers destined for Carlinville or Alton will be provided separate motorcoach service upon the train's arrival in Springfield. Carlinville and Alton passengers will not be able to make connections to Train 313 in St. Louis. Alton passengers can make reservations aboard Train 301 to make connections with Train 313, destined to Kansas City and intermediate stations. Northbound Trains 300, 302, 304 and 306
On April 16 - 24, May 1 - 9 and May 16 - 24, all passengers boarding at St. Louis who are destined for Springfield and points north will be provided express motorcoach service directly to Springfield, where they can board the train. Motorcoach service will depart 20 minutes earlier than the train's normal departure time. All passengers boarding at Alton and Carlinville will be provided alternate transportation to Springfield, where they can board the train. Alternate transportation will depart Alton one hour and 15 minutes earlier than the times shown in Amtrak printed timetables and 50 minutes earlier from Carlinville. All passengers scheduled to board the train at St. Louis who are traveling to Alton or Carlinville will be provided alternate transportation, which will operate according to the schedule of Train 304 and 322. April 16, May 1 and May 16, 2012 Exceptions: Train 300 will operate according to the normal schedule on April 16, May 1 and May 16, 2012.

April 25, May 10 and May 25, 2012 Exceptions: Passengers scheduled to board Trains 300 and 302 will be provided motorcoach service from St. Louis to Springfield on April 25, May 10 and May 25, 2012.