The northern entrance to Riverbank State Park sits at the end of West 145th street, and leads to a bridge that spans the West Side Highway, the Riverside Park bike path and two sets of train tracks. Riverbank itself sits atop the North River Wastewater Plant, which processes 125 million gallons of Manhattan sewage every day. At the end of the bridge is a set of stairs that takes you down from the Park complex to the level of the river. A few hundred feet from these stairs, on a small grassy verge by the fenced-off train tracks, I found my son and his friends.

This grassy space has become a favorite spot for my son to indulge his hobby of "railfanning." I had no idea what railfanning meant the first time I heard the term. It sounded dangerous, the kind of teenage kick that leads to the emergency room and a broken arm or fractured collar bone. Presumably, to railfan involved moving fast, taking risks and, if you weren't careful, eating it in front of your friends.

In fact, railfanning involves long stretches of doing nothing. The speed and movement is provided by massively heavy train engines, not the people watching them. Maybe the railfans jump up and down now and then, but mainly they expend their energy by pointing their cell phone cameras and commenting on what they see.

A railfan (or is it railfanner?) is a train enthusiast--the type of person who, in Britain, is called a train spotter (or, more derisively, an "anorak.") My son is an avid railfan, and as my wife and I feel a certain amount of worry at the thought of him lingering next to train tracks on his own, we have had to become reluctant railfans ourselves. We take turns hanging out on the grass verge, looking at the sky or checking our phones, wondering when the train is finally going to speed by.

This particular spot is pleasant enough. The bike path is well-maintained, the Hudson River is visible across some playing fields and a stretch of park, and the whole area gets a lot of light on sunny days. But it is close to a building containing the largest lake of shit on the Upper West Side. The air has an acrid, acid smell that isn't strong enough to send you running, but slowly seeps into your lungs and leaves an unpleasant salty taste in the back of your throat. My son and his friends don't seem to notice it, but it does loom large in my mind. Unlike them, I'm not excited at the prospect of the 4:10 from New Haven thundering past us 15 minutes from now. I'm just bored, ashamed that I'm bored, and slightly bewildered by the whole thing.

Before having kids, I pictured fatherhood as a sort of movie montage of shared moments. I pictured my kids and I listening to the Clash together, laughing at old episodes of Monty Python, or staying up late reading from The Lord of the Rings. In other words, all the things that interested me as a child. (You'll notice there's no throwing the ball in the front yard stuff, which should give you an idea of the type of kid I was.)

Parts of this wistful fantasy did come true--my sons will occasionally close Spotify to listen to "Give Em Enough Rope" on the stereo--but far more common is something I didn't anticipate: that in order to look after my kids, I would have to take an active interest in those things that fascinated them. They don't care what I like. They want me to like what they like.

Sometimes this means taking the light rail from Trenton to Camden, NJ, or spending three hours on the A train to go out to Far Rockaway and back in the middle of winter. Sometimes it means taking the subway out to Broadway Junction, in Queens, and walking from the A train platform to the L train then over to the Z stop, watching the trains come in but never actually getting on them.

This can be a drag, but sometimes, when I find myself bored or resentful, I have a premonition. One day, I think, my kids won't want to hang out with me at all. One day, I'll envy the amount of time we're spending sharing this hobby, even if I didn't choose it.

So I find myself by the side of the railroad tracks, next to a sewage plant, waiting for a commuter train. When I hear it coming, like as not, I'll pull out my iPhone and take a video of the engine as it speeds past. In case, for some reason, my son is looking the other way.