Metra F40C's were originally designed for Milwaukee Road commuter service in Chicago. The fluted silver panels were installed to (somewhat) match the fluting on the bilevel commuter cars in service at the time. The F40C's were not the same as Amtrak SDP40F's. When Metra took over the service and repainted the locomotives from Milwaukee Road colors into Metra livery, they removed the rearmost panel and painted on the Metra logo in large billboard fashion.
[
www.rrpicturearchives.net]
[
commons.wikimedia.org]
[
en.wikipedia.org]
Hope this helps...
--Sasquatch