The Pennsylvanian from Newark NJ to Pittsburgh PA has a luggage car with hooks for bikes
The Northeast Regional from Washington DC to Newark NJ does not have a luggage car, so at the conductor's instructions, I boarded and strapped it down where wheelchair passengers are often sat. Not sure what I would have done if there was a wheelchair passenger in that spot - maybe find another car with an empty spot.
This Ikea bag/backpack holds 2 x 17 liter panniers plus a handlebar bag plus 2 x 4.1 liter Ortlieb fork mount packs. I might be able to squeeze 4 x 17 liter panners but I haven't tried.