.01 on the BNSF industrial track at the corner of Halsted Strett and Cermak road in Chicago.
The Minnesota Zephyr line now isolated is sitting until june 1st. Covered with snow I have no idea if it has mile markers anymore but the line is said to be 5.9 miles long. Tracks through downtown Stillwater were ripped up in the summer of 05 or 06. It used to be a BN line. The track just south of downtown is in no shape to run a train on.
One railroad that could be very intersting to figure out mile markers for is the Wisconsin Southern. They run on a lot of former (historcally) Milwaukee Road and Chicago & Northwestern trackage in Wisconsin and northern Illinois.
The ex-Chicago Great Western is another railroad whose mile markers started in Chicago and worked west. Most of the mainline has been abandoned but small segments survive near Byron, IL (power plant) and in Iowa.
CC