Last week my wife and I took a round trip on the Missouri River Runner between St Louis and Kansas City. I was surprised to see that these trains were running with seven coaches, only three of which were in use. I asked the conductor about it. He said that late in 2016 the UP began requiring that Amtrak run at least 30 axles on these trains so they wouldn't disappear from dispatcher displays.
He also mentioned that Amtrak pays the UP by the car (or maybe by the axle) so the extra unused coaches required by the UP were generating extra revenue for the UP.
We enjoyed the round trip. It was a great way to spend a day.