As the largest commuter railroads in the U.S., Metro-North (and its sister railroad the Long Island Railroad) are victims of geography. They both serve large portions of the New York City metro area, but not all of it. Something is standing in the way: the Hudson River.

If you want to go to, say, Newark Airport or Princeton, NJ, your only non-driving mass transit option is Amtrak with pricey and infrequent service. Want to visit family and friends in Rockland County, NY? You have to drive.

All of New York City’s commuter lines run on a hub-and-spoke model with trains running into and out of the city’s two stations, Grand Central and Penn Station. But what if Metro-North could conquer the Hudson and offer “through service?” 

World-class cities like Paris and Tokyo have commuter trains that run through their metro areas. Why can’t New York?

[RELATED: Riding Metro-North in CT: What to know about fares, train times, stations]

Imagine being able to grab a train in Bridgeport and, without changing trains, end up in New Jersey. That one-seat ride would open up a world of possibilities of new jobs, college attendance and recreation.

But how to get across the Hudson? Well, there are a couple of options… one that’s been engineered for the future and the other which could be implemented much quicker. But they both present serious challenges.

Up and over the Hudson: You may not realize it, but the new Mario Cuomo (formerly Tappan Zee) Bridge from Tarrytown to Nyack was designed to eventually be able to carry commuter trains. The trains would run in the bridge’s median, between north and southbound auto and truck traffic. That space is still available, but there is no rail service (yet) for a couple of reasons.

First, to climb from the near water-level height of the Metro-North Hudson line up to the bridge would mean ascending almost 140 feet. Trains can’t climb hills with more than a 2% grade, so that embankment would have to start 1.33 miles before the bridge. That would chew into a lot of expensive river-view real estate, be very expensive and take a long time to build. So, no trains on the Tappan Zee… for now.

On the other hand, if Metro-North ever built an east-west spur along Interstate 287 connecting Rye to Tarrytown, that height difference wouldn’t be a problem. But that’s another pipe dream… for now.

Under the river: Metro-North trains from Connecticut will soon be able to take the Amtrak route across the Hells Gate Bridge and into Penn Station. From there they could travel under the Hudson and on into New Jersey. But this plan also has problems.

First, Amtrak and NJ Transit have plans to expand service at Penn Station, pretty much maxing out its capacity even after expansion of that station.

Even if there was room, Metro-North’s trains run on different voltage (12 kV AC 25 Hz) than the Northeast Corridor line in New Jersey. In 2009 when NJ Transit ran experimental trains from Connecticut to Giants Stadium, their dual-mode locomotives ran electric in New Jersey and then diesel in New York and Connecticut.

The bottom line: through-running makes sense, but like so many good ideas it would be really hard (and expensive) to make happen.