There are multiple reasons for "bustitution" rather than detouring of Amtrak trains. First, Amtrak supplies its own crews. The ranks are pretty thin, especially on long-distance routes with one train a day in each direction. If a detour causes a crew to "die on the law", Amtrak may simply not have a substitute crew. Second, as noted earlier, the freight railroads have abandoned much of their excess capacity. There simply are not detour routes in good condition. Third, on a detour Amtrak will need a pilot, and especially when traffic was heavy just a couple of years ago, railroads may not have had the qualified crew members Amtrak needed for pilots. I know some planned Starlight detours via Bieber were canceled due to crew shortages three or four years ago.
I've personally been on five Amtrak detours that I can quickly recall. The first was from Birmingham to Atlanta on #20, via CSX rather than Southern, due to signal work on SR. The CSX line has now been abandoned.
The second was a detour from Elkhart to Kalamazoo due to Amtrak track work between Porter, IN and K-zoo. This is still available today, I think.
The third was a 1988 detour of the Texas Eagle from Big Sandy, TX to Flatonia on the former Cotton Belt. That detour would still be possible today.
The fourth one was an eastbound Empire Builder detour over BNSF via LaCrosse and Aurora due to labor problems on CP (Soo Line). That detour is still available, and has been used in the recent past.
Finally, I rode one of the Starlight detours over Tehachapi in 2008.
So it all depends. In some cases, detour routes are still available and in good condition, but in many others alternate routes either no longer exist, or shortages of qualified crews make them infeasible. The railroad network has changed a great deal since 1971.
Randy Resor, aka "NellieBly" passed away on November 1, 2013. We honor his memory and his devotion to railroading at railroad.net.