To answer Astoria_amateur: no, don't tap. I know that feels impolite, but it's safer than setting off someone who's already on edge. I've lived near the 34th St express stop for the better part of a decade, and the top-of-stairs cluster is a year-round thing, not just tourist season or rush hour. The real problem is a mix of station design and phone habits: the stairwell opens onto a landing that feels like a safe place to stop because it's a few feet wider than the stairs, but it's exactly where everyone behind you needs to go. I've watched whole lines of people compress into a bottleneck because one person stops to check a notification the second they get signal.
What I've found works, most of the time: slow down before the top step so you have a little buffer, and say 'coming through on your left' in a calm, non-confrontational voice. Don't touch them. Even a light tap can escalate, especially with people who already feel crowded and defensive. If they don't move, I step into any clear space or wait for a gap. If I'm behind someone with a suitcase or stroller, I give them extra room because they've got more to maneuver.
I did once see a woman stop at the top of the 23rd St exit to photograph the station sign with her phone, suitcase kicked out almost sideways. I said 'please keep the landing clear,' and she did move, but her companion rolled her eyes at me. Some people genuinely don't realize they're a hazard; they think a few seconds doesn't matter, but in a dense station those seconds stack up behind them. I'd support the MTA painting 'keep moving' at top landings, though I'm not holding my breath. Until then, I treat the top of the stairs like a merge lane: pick your line, look ahead, and don't be the person parked in it.