With this timetable, the Empire Builder permanently lost its observation car, as it was no longer a part of the train’s summer consist. The eight-seat observation lounge in those cars contributed little to the enjoyment of passengers, most of whom would rather be in a dome or food service car if they wanted to leave their seats or rooms.
Click image to download an 14.5-MB PDF of this 24-page timetable.
Mainly the round-tailed observations were part of the streamlined train aesthetic, an aesthetic that was fast disappearing as Union Pacific, Santa Fe, and other railroads dropped observation cars from even their finest trains. Moreover, some trains, including the Empire Builder, would soon be led by boxy Diesel locomotives, thus being unstreamlined at both ends.
The Empire Builder was still combined with the Afternoon Zephyr westbound and the North Coast Limited eastbound in the non-summer months (basically October through May), while they operated separately during the summer. Eastbound the summer North Coast Limited ran just 15 minutes apart from the Empire Builder but westbound the Afternoon Zephyr ran 1-3/4 hours after the Empire Builder.
Strangely enough, SP was the holdout when it came to observation cars and similar rolling stock. The Coast Daylight had a round end parlor observation right up to the end (albeit 2 days out of 3) and the San Joaquin Daylight had a round end coach observation, although that sometimes ran at the front of the consist because SP did not bother to turn the train at L.A. The blunt end 10-6 sleeper originally used on the Sunset was used on the Cascade at least into the late 1960s.
As for streamlining, I seem to recall reading somewhere that it was not the smooth contours that made for more cost efficient operation, it was the lighter weight of the cars themselves.