A reader disagrees:

The Amtrak analogy is wrong and there is a danger to using that analogy.
Amtrak involves a dead inefficient industry/technology. GM involves a dying player and business model (US industrial unions). An important difference is that GM has robust competitors within the industry.
The danger is that the left has been trying for years to brand the auto industry/technology as a dying one (ironically while touting the vitality of rail). If you legitimize that, you set the left up to commence an orderly consolidation and winding down of the auto industry as a whole.
As with everything, the left gets to have it both ways: using our (correct) view of rail as dead; plus the incorrect analogy to justify euthanizing the auto industry, all the while denying the morbidity of rail.
There may be no direct analogy. The presence of competitors suggests a postal analogy, but this is a poor fit.