Empire Builder Storage Mail Car

I am looking for information on a Storage Mail car that was assigned to the Empire Builder in either 1954 or 1955. The car operated between Chicago and St. Paul in the Milw Road Morning Hiawatha and was transferred to the Empire Builder in St. Paul for the trip to Spokane. Need dates this car operated and understand the car was always GN as they did not want foreign car breaking up the Empire Builder colors. Also understand the car ran monday through saturdays only and thr cars returned east in the Fast Mail or combined Western Star /Fast Mail. Need dates these cars operated if available. THX.

The Great Northern Historical Society site http://www.gnrhs.org has a reference sheet on the Empire Builder 1955-59 (available online) which shows the Chicago-Seattle storage mail car operating daily westbound throughout this time period. Eastbound, the car ran Seattle-St. Paul, and did not depart Seattle on Saturdays or Sundays.

Another reference sheet (No. 244, from September 1996) gives the history of the Empire Builder from 1960-70. It probably gives the remaining operation dates of this car.

I’ve seen photos of westbound Morning Hiawathas (MILW train No 5) carrying Great Northern mail storage cars in their consists dated as late as June 1965. However, in photos of the same train dated June 1967 and later, the GN car is not seen.