A more important aspect may be that in most cases, Amtrak
does not have passenger facilities along the detour route
and therefore cannot sell any tickets for people to board
or detrain on the detour route.

So, once the train leaves the base route, it loses all
possible revenue from passengers who would have boarded or
detrained on the segment that is being skipped.

Buses can still serve some of the points on the base route,
even if a train cannot operate on that route.

On most long-distance trains, there are already passengers
on board for destinations on the route that has suddenly
become unusable due to a sudden problem. Similarly, there
will be passenbers who have already bought tickets to board
on the currently unusable route segment. Detouring the
train may get the equipment around the problem area, but it
does nothing for these passengers.

And, unfortunately, the U.S. does not have a dense enough
network of passenger routes that it is easy to detour around
just a short route segment. Years ago in Europe I was on a
local train that was blocked by a rockslide. The train backed
to a nearby junction point (only a few miles) where I was able
to transfer to another train going on another route -- but in
the same general direction. I still got to my final destination
with only about an hour's delay.

-- Ernest