Just an FYI...the ClubAcela in WAS closes at 9PM. They've pushed me out the door a couple of times while I was waiting to board #66. I think it was perhaps 5-6 years ago, when I had to spend the next hour or so in what passes for 'chairs' at WAS. Without a doubt, they are the most uncomfortable chairs I've EVER sat in, especially for a couple of hours, when #66 was running late. Now those chairs have been introduced at other stations like PGH, etc. Any place with those uncomfortable chairs (there's less than 1/4" foam with a vinyl covering on them, and uncomfortable metal arm rests to separate you from the person next to you) would be my LAST choice of where to spend the night. Maybe that's intentional to keep the homeless from sleeping in the chairs. HOWEVER, there's Amtrak trains leaving WAS about every 2 hours during night, plus any late arrivals, so there's more than just 'undesirables' waiting for the train, too.
That leaves PHL. They still have the original 100-year old benches in the waiting area. I think their ClubAcela also closes at 9PM. However, someone horizontal on one of those benches is likely to get one of the RR cops to wake you up and tell you to 'move along', etc.
About a year ago, I wanted to take the westbound Cardinal, like you. Living in New England, I've ridden eastbound a fair number of times, and connect to #66 at WAS as they have the best food court, in my opinion vs PHL or NYP. Plus, by the time #50 reaches PHL, the vendors would be closed. So, I did a 'hotel check' in the WAS area online and had to limit my choices to one that was within a block or so of a subway stop...any subway stop. I found one in Alexandria pretty much across the street from the subway (elevated) station, and provided a full view of the Amtrak trains as well. It also has the distinction of being built on top of the late, great, Potomac (Pot) Yard. Unfortunately, it was quite pricey. There's two hotels there, and I walked into the wrong one. Having been a night auditor at a national hotel chain 15 years ago, I naturally asked the clerk what the walk-up rate would be at 7PM when I got there. He quoted $125. By 7PM or so, they know how many rooms are likely to go unfilled that night and most clerks are given some leeway on what price to quote, just to fill the bed and make a couple dollars. Sort of like flying 'standby' in days past. But I've had a number of business trips years ago to WAS and one time found that my 'usual' hotel (a Travel Lodge near the Iwo Jima Monument in Alexandria) was booked solid due to a convention. Other times, I found that reserving rooms at the government rate (I was on gov't contracts) was more expensive than a walk-up while I was checking in one night. You can't win.
So, finding something at BAL would be workable, hopefully on the trolley line. Heading towards the south, there's some 'high priced' (I think) chain hotels near the college, stadium, and the B&O Camden Yards station (end of the line for B&O line MARC commuter to WAS). Changing trolleys at Camden Yards station, you can ride to the BWI airport, but I don't recall seeing hotels within a short walk from any of the stops.
However...as has been discussed on another site, there's a Holiday Inn or something like that directly across the street from the Newark Penn Station (Amtrak) that's very reasonable. I'm contemplating spending a night there to maximize my NJ Transit 'exploring' time rather than down and back to Massachusetts the same day as I have been doing. It's GREAT being retired...
