I posted an explanation but it didn't "take," so here goes a second try:
"The Mercury train sets were designed by the noted industrial designer Henry Dreyfuss, and are considered a prime example of Art Deco design. "
"A fourth train, the James Whitcomb Riley between Chicago and Cincinnati, used the same design for its train sets and is considered part of the Mercury family, although it did not bear the Mercury name. The Riley debuted in 1941 and lasted into the Amtrak era, though no longer a streamliner" "
Amtrak was "Founded in 1971..."
"The Riley was retained, although it was no longer a streamliner. In 1971, Amtrak combined the Riley with the Chesapeake and Ohio Railway's George Washington."
"Amtrak took over intercity passenger rail service on May 1, 1971. Amtrak kept the George Washington, including both the Washington and Newport News sections. It was combined with the James Whitcomb Riley on July 12, 1971 to provide through service to Chicago"
All quotes are from Wikipedia.
But Hot Water, you might be right. Carefully re-reading these statements, I wonder if the author of these articles didn't accidentally blend the literal locomotive named "James Whitcomb Riley" with the passenger service of the same name. I had understood the original statement to mean that the locomotive itself was removed from streamliner service but was still used for other duties. I cannot find any information on when the locomotive itself was retired.
Meanwhile, I had to add some photos of this magnificent steam locomotive:
