If you arrive on a sleeper connecting to an all coach train, then you should be able to use the Lounge, it part of the reservation to get from point A to B. If you arrive on Coach connecting to a Sleeper there is no problem, so why would the passenger be treated such on the reverse?
Shouldn't. Your purchase for a BC or Sleeper is supposed to give you total access to the lounge no matter what your ticket says for your connecting train. That policy is still stated on the Amtrak page:
https://www.amtrak.com/servlet/ContentServer?c=AM_Content_C&pagename=am%2FLayout&cid=1241245658203&WT.z_va_evt=redirect&WT.z_va_topic=Station%20Information&WT.z_va_unit=Metropolitan%20Lounge&WT.z_va_group=Station%20Details
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Metropolitan Lounges are available in Chicago - Union Station, Los Angeles - Union Station and Portland, OR - Union Station to sleeping car passengers, business class passengers with a same-day travel ticket (departure or arrival) and Amtrak Guest Rewards Select Plus or Select Executive members. Relax and enjoy comfortable, quiet lounge seating, complimentary non-alcoholic beverages, snacks, and internet access. Attendants are available to assist with reservations, ticketing and local information."
As I said above, I didn't have any problems switching from the Pere Marquette Business Class to the Hiawatha, which is all coach, using the lounge during the layover.
With so many problems, I plan to print out the policy and have it with me every time I'm in that scenario.