Are the SL and Cardinal worst-performing because they are tri-weekly or are they tri-weekly because they are worst-performing? Or both?
Probably a bit of both. They were probably the ones selected for tri-weekly service because of the lowest ridership - and at at time when overall ridership was low and Amtrak had to make cuts.
But the passengers just don't wait to travel till the day the train leaves. Many of them find other travel modes, and ridership per train is probably the same or worse as you have to fit out and home into an inflexible schedule. Especially with rising ridership overall it definately should be possible to market a daily train.
However the big issue is cost. One daily train is a very expensive way to run a line to start with as station staffing, layover facilities, catering etc. all has to be in place for a once a day event. All these costs remain almost the same for a tri-weekly train and you can add problems with layovers for staff and rolling stock. With three departures a week, at least once a week there has to be an extra 24 hour layover at the terminus. If this is the home terminus of the staff, fine, but otherwise it costs, and so does inactive equipment just sitting there. Sending staff and equipment to serve another route from that terminus just throws chaos into that schedule as well. All in all a more complicated and less efficient operation than daily trains.
So even if they were the poorest performing routes to start with, the tri-weekly schedule makes it virtually impossible for them to perform well :huh: