A hypothesis about US infrastructure/politics:
Amtrak continues to exist for the same reason the Democratic Party exists—to make the alternatives (respectively: driving/flying, the Republican Party) look reasonable.
(If American governance was sensible, the feds would have dropped about $1Tn on building out a modern, high speed, electrified, grade-separated network no later than 1980 and would now be in a whose-trains-are-faster-and-more-punctual race with China.)