An issue that even most people who consider themselves interested in mass transit don't pay enough attention to are the severe problems that US transit agencies seem to have in cooperating with each other. Both the New York and DC metropolitan areas, for example, are cursed with multiple commuter rail operators (MARC and VRE in DC, NJ Transit and LIRR in NYC) whose trains dead-end in the middle of the city rather than through-running into the jurisdiction on the other side. This creates some bad problems simply in terms of space management, but it's also poor transit service. Consider Aaron Weiner's piece about efforts to bring some development to Ivy City in DC, an area that's underserved by mass transit in a particularly egregious kind of way: "taunted by transit it doesn’t benefit from: the Amtrak tracks that box in a neighborhood that lacks easy access to any Metro or intercity rail stations, and the whizzing cars along New York Avenue that rarely have occasion to pull off in Ivy City."
The thing is, you couldn't even build Metro access to Ivy City in any kind of reasonable way. But those Amtrak tracks are also used by the MARC Penn Line as it passes from New Carrolton to its final destination at Union Station.
Now as long as the MARC ends at Union Station, adding an infill MARC station in Ivy City wouldn't be very useful to anyone. But if it ran-through to the VRE stations at L'Enfant Plaza, Crystal City, and Alexandria then that'd be pretty useful transit. You'd have connections to all the Metro lines and access to several different employment centers. It's not "easy" (see here and here) to implement through-running, but it's a lot easier than giant new construction projects. It's something that could both substantially increase the value of existing mass transit infrastructure, and also open up the doors to relatively cheap infrastructure additions like this hypothetical Ivy City infill station.