My 1928 RR Atlas marks the route as Pennsy. My 2005 SPV Map shows Pennsy as the original operator of the Vandalia, including the TH&P. I learned of the TH&P corporate ancestry from Bridge Hunter.
Note the line on this Pennsy map going between Terre Haute and Peoria.
|Map 9 from Mark D. Bej's CentHist, Copyleft|
|An excerpt from the above map at downloaded file resolution.|