I spent a year commuting every day from PHL-NYP and back. I've found that Amtrak has been retrofitting coach class cars with business class seating, and that almost every train has at least two such cars - one actually serving business, and at least one somewhere else on the train.

As someone else said they frequently cover up "coach" on the outside of the train with a taped "business" sign, but not always. As I get on a train, I usually look for a dirty tape outline around the word "coach" and I get on that car for more legroom.