Amtrak has (most recently, I believe, with the Crescent) changed a schedule to improve on-time performance. It hasn't worked, and the train operates late under the new schedule. (I have no evidence at hand to back up my assertion and don't intent to search for any; it's just what I've read.) Put another way, a railroad that doesn't try to meet a negotiated Amtrak schedule will not try to meet a negotiated Amtrak schedule.

With the Southwest Chief, zephyr17 has explained why this might be a temporary situation. If it is (and I sure hope it is), then lengthening the schedule would be a mistake.