How long were Milwaukee Road's streamlined 4-4-2 Hiawatha locos kept in service? Did they keep their streamlining throughout their life? Did any survive for excursion runs or in museums?
To answer in reverse order:
No, none were saved. All of them were scrapped.
AFAIK none of them were ever de-streamlined; The 4-4-2s were streamlined from the start, it wasn't something added after they were built and later removed (the way some NYC Hudsons were built as regular engines, then streamlined, then de-streamlined.)
After the streamlined 4-6-4 engines arrived, the 4-4-2s worked on mail trains, commuter trains, local trains etc. until they were scrapped...don't know the exact year, but sometime in the early-mid 1950's.
Nos 1 and 2 were purpose built for the Hiawatha; #3 and 4 were almost identical but built for Chicago to Milwaukee service, to "cover" for 1 and 2, and other such purposes. They did see general passenger service but also worked the Afternoon Hiawatha even after the streamlined Baltics arrived until the diesels took over. Then the four later saw service on the Midwest Hiawatha, again, right up to diesels.
#3 was retired first, to provide needed parts for its sister engines: 9-15-49.
#1 was retired 11-9-51
#2 was retired 11-9-51
#4 was retired 6-8-51.
My source is Jim Scribbins's book The Hiawatha Story.
They all were looking pretty ratty towards the end as parts of the streamlined shrouding had been removed and reinstalled enough times (for maintenance purposes) that the original smooth appearance was largely lost.
Dave Nelson