The MARC train, run by Amtrak, is the commuter train that runs from DC through Maryland, I've been taking this train between Baltimore and DC for almost two years and have developed a strategic approach to seating that illustrates the Prisoner's Dilemma when trying to optimize leg room.
For the risk averse, you can go ahead and sit in the typical front-to-back seating arrangement and guarentee an airline coach seat's worth of seating. But for us with a high risk, high reward mentality, each car on the MARC train has some front-to-front seating configuration. In this seating arrangement, a pair of seats face another pair of seats. This four seat arrangement is usually situated in in each of the corners of the car. (There is one car configuration that has two four seat groupings in the center of the car.)
It's possible to enjoy the whole commuter trip between Baltimore and DC stretching your legs out with no one sitting across from you. Your seat selection becomes crucial in increasing the chances of this leg room.
It's easiest if you are the first to sit in a four seat pod. Take one of the seats next to the window. The next person to join you will most likely sit in the aisle seat across from you. (Apparently, people don't like to sit next to each other if they can avoid it.) Unfortunately for them, they've only improved your chances of extra leg room. The third person to the pod will most likely sit across from that person (and next to you), rather than climb over everyone's legs to get that seat across from you.
At that point, unless the train is crammed full a fourth person will prefer other seats over getting into that last spot. Sometimes, your fellow passenger will help you out by putting their backpacks and laptops in that empty seat. Now, not only does the person have to climb over legs, they have to ask someone to move their stuff. Even if someone does all that and ends up across from you, you've already enjoyed a good part of your trip with extra leg room.
But what if you are not the first person to sit in the four seat pod?
In that case, your best option people are only sitting in the aisle seats. Climb over them and get one of the open window seats. You're basically just as well off as if you'd taken that seat before anyone else had taken a seat.
But what if someone has already taken the window seat. Well, then you need to suck it up and sit next to them and hope that the next person will sidle over to the remaining window seat. If you take the catty corner seat, your extra leg room won't last the train's departure from the station, as someone will inevitably sit across from you.
Worse case scenario is when you've taken that first window seat, and then the next person inexplicably takes the seat across from you! I have no idea, why someone would do this, whether spitefulness (to you) or politeness (for the next people to site down) or if some people just really like the window seat no matter what.
You could move over a seat, but it's probably a lost cause. You gambled and this time it just didn't quite pan out. But like I said, "High Risk, High Reward" on the commuter train.