The platform at DC's NoMa Metro station is one of the best places in the US to spot passenger trains. From there, anyone can see a steady stream of Acela high speed rail trains, regular Amtrak trains, MARC and VRE commuter trains, and of course Metro trains, going or coming from Union Station, which is the 2nd largest intercity rail station in the US (after Penn Station, NY).
All these pictures come from standing on the NoMa platform for a period of only about 20 minutes, a week or two ago.
Here's a quick map showing where the two stations are. Most of these pictures are from NoMa, looking south towards Union.
Let's start with Acela. The US Capitol is just south of Union Station, so you'll see its dome peeking above throughout this thread.
Here you can see the Metro platform.
Now a few regular Amtrak trains.
You saw that MARC train in the last picture. Here are more. MARC is commuter rail in Maryland. Two of its lines go up to Baltimore, and the other goes over to Gaithersburg, Frederick, and the tip of West Virginia.
Now VRE, which is commuter rail to Virginia. Lines go to Alexandria and then Manassas and Fredericksburg.
One to leave you by, MARC, an electric mover, and Metro: