The Wisconsin DOT (PDF document) does list how many people ride the Hiawatha last year, and that was 397,518 passengers. That's for 6 daily round-trip runs and 1 additional Monday-Saturday round trip run.
To do the math, first divide that 397,518 in half to 198,754 actual riders (roughly, there are probably a few that don't make the round trip). Then divide by 52 weeks to get 3,822 riders per week. Divide again by 48 round trips per week to get just under 80 riders per round trip.
How does the trip time via train compare with the probable time via bus? The level of ridership cited would certainly seem like enough to justify some sort of mass transit connection; I suspect busses, run somewhat more frequently, might be better than trains but that depends a lot on the relative transit speeds.
Amtrak does have a few routes that do very well, and a moderate number of routes that do reasonably. There are many routes for which new infrastructure construction would make little sense, but for which use of existing infrastructure is quite reasonable. This would seem like it might be one of those.