What is the smallest radius turn?

[DowneasterPassenger]

A post on another topic got me wondering--

What is the smallest radius turn that an Amtrak train can take?

How slow would it have to go?

Where is the smallest radius turn in the Amtrak system? The smallest one that passengers experience and the smallest one that the cars might run on?

 

[Joel N. Weber II]

George Harris's post in this thread has a bunch of answers.