by RpR
When was the last UP articulated/semi-articulated train-set locomotive retired? (Pre E units)
Short answer: 1953. That's when the City of Denver M-10005 and M-10006 were retired. But its actually more complicated than that:
M-10000, City of Salina, was built in 1934, entered regular service in 1935, was retired in December 1941 & scrapped in February 1942.
M-10001, City of Portland, was built in 1934, entered regular service in 1935, & was retired in 1939. The power unit was rebuilt as an additional booster for the CD-07 locomotive set and the cars were stored until scrapped in 1941.
M-10002, City of Los Angeles, entered service in 1936. This train had 2 locomotives that could be uncoupled from the rest of the articulated train. Styling was still similar to M-10000 and M-10001. M-10002 replaced M-10001 as the City of Portland in 1939 until 1941, and was then a Portland-Seattle train from 1942 until 1943, when it was retired. A few of the cars were added to M-10004 while the rest of the train was scrapped.
M-10003 was a spare set of power units delivered in 1936 matching those used on M-10005-6. They later became the CD-07 set of power units. There was no M-10003 train.
M-10004, City of San Francisco, entered service in 1936. The articulated train was virtually identical to M-10002, but the locomotives matched the M-10003 spares and City of Denver locomotives. In 1938, M-10004 was replaced on the City of San Francisco route and became a second City of Los Angeles train. In 1939 the power units were rebuilt into extra boosters for the City of Denver M-10005 and M-10006, and from then on the M-10004 set was usually pulled by the CD-07 set or E3s. In 1941 the M-10004 set replaced M-10002 as the City of Portland until its retirement in 1947. The M-10004 substituted for the City of Denver trains wheile they were renovated in late 1947 & early 1948, then the M-10004 cars were put into storage until scrapped in 1951.
M-10005 & M-10006, the City of Denver twins, entered service in 1936. These trains weren't truly articulated but were a mix of single and 2-unit articulated cars. Aside from the renovation in 1947-48, they remained in City of Denver service until retired and scrapped in 1953.
E2-powered City of Los Angeles and City of San Francisco trains arrived in 1937-38, and additional E6-powered City of Los Angeles and City of San Francisco trains arrived in 1941. These trains weren't articulated, though they did include a few 2-unit articulated cars. These were the last to be ordered as complete trains; from then on UP bought standardized locomotives and cars that could be used in any train as needed. The cars of the E-unit powered trains ended up being mixed into the general pool, and a few survive today.
One side note: from December 1956 to September 1957 Union Pacific leased GM's articulated Aerotrain and used it as the City of Las Vegas, if you want to count that.
Source for all: The Union Pacific Streamliners by Harold E. Ranks and William W. Kratville.