Last fall, if I recall, Amtrak stopped 'offering' a number of different routings from my 'home base' in Springfield, MA. In particular, in addition to various 49/449 options to CHI, the site previously offered 141/43/29 via NYP & PGH as well as 141/29 via WAS. The advantage to both those now 'missing' routings is that they were usually $100 or more cheaper for a roomette than the direct Lakeshore Ltd route. Amsnag doesn't offer the extra routings, either.
Fortunately, I can still book whatever routing I choose, but I have to use Amsnag on a segment by segment basis and then decide which way to go for the least money. SLC-FTW is likely another of those 'reduced number of offerings' on the Amtrak website. It's a great way to drive away potential passengers, if you ask me.
Fortunately, using 'multi-city' still works, but it may balk at trying to book 'too tight' connections. It did that to me a couple years ago, so I booked the 2nd leg as a separate reservation and barely made the connection.