Every time I travel on the London Underground, my ears “pop” like they do at high altitude (though not as severely). Does anybody know why this happens?
I’ve only been here about five weeks, if that makes a difference.
This happens to me quite a bit, and I’ve been in London 5 years. It definitely affects me more on certain lines like the Central and Northern, and much less on the District or H&C.
Based on that, my theory is that it’s caused by a combination of altitude changes and pressure waves. Tube tunnels do go up and down, and there can often be pressure variations because of changes in altitude. I think there’s also often a pressure wave that’s caused by the train entering an enclosed space like a tight tunnel from a relatively open space like a platform, or by another train coming down a tunnel in the opposite direction.
If you’ve been here 5 weeks, you should know it’s called the Tube! 
It’s not caused by altitude changes - Tube track gradients are pretty shallow. It’s just the same effect that causes your ears to pop when you go through any narrow train tunnel. The train displaces air as it moves, and when the tunnel is too narrow for the air to move out of the way easily, it creates a pressure wave. You feel the increased pressure on your ears.
The reason it’s not a constant effect is that the tunnel width varies, and there are side tunnels and alcoves that release the pressure in places. Also trains coming the other way, and air currents (no doubt you’ve felt the strong breeze on some platforms) can add to the effect.