All trains from or to Virginia change engines. An announcement is usually made not to go up to the station. But if you do it... City of Miami is correct that there seem to be two ways down to the platform from the concourse in the station. One has a queue of passengers waiting for the go-ahead. They board about 10 minutes before departure. Yet I was able a few years ago to just go around that queue to a different set of stairs. Note: the useful shops in the station are not especially close to the concourse, and you will probably not find a newspaper on sale, the same as most airports nowadays. So since that trip, I heed the announcement and just enjoy the platform, or stay on board as it goes dark.
An unfortunate incident of a passenger leaving his bag on the train and just missing its departure, then hanging on the side through the tunnel at WAS shows it's best to keep an eye on whatever Amtrak staff is at an open door, or on the platform by an open door, no matter the station. And vice versa staff or cameras should monitor a situation like that. (He survived but was shaken up.)
I'm curious when the dual mode locomotives will appear, but there's no sign they're being built yet that I know of. I guess the one spotted in Savannah was an ALC-42, not dual mode. Those models have numbers 300 to 424, says Wikipedia, and the latest spotted here was 342. The numbers for the dual mode ALC-42E's are "TDB" at Wikipedia.