So when you are riding in a train and looking out the window (or not), one of the notable things that goes by outside is other trains.
Other trains are notable, in particular, because unlike most of the other things that go by outside, other trains are, a non-negligible fraction of the time, not standing still (relative to, say, the train tracks), but are moving. And also are often real close-by.
Even if the other train is standing still, since there’s a reasonable chance that it might not be, you can’t reliably tell that it is, just from what you see out the window when it’s filled up by the other train. It might be standing still and your train is moving, or your train might be moving while the other one is standing still (the actual case in the current hypothetical), or both trains might actually be moving (relative, again, to something like the tracks).
If the other train is moving, in the same direction and at about the same speed as your train, you can see out of your window and into the window of the other train, as it slides slowly forward and back due to small changes in relative speed, and sometimes there are other people over there that you could, say, wave at. Unless you are too shy at the time. (Which is fine.)
If the other train is moving in the other direction, then there is great rush and zooming and blurriness, because the other train looks like it is moving very fast indeed, at twice the average speed of the trains, or at the magnitude of the difference of their velocities (equal under the assumption that they really are moving in exactly opposite directions, basically). Which is very fast if both trains are moving somewhat fast. If both trains are moving slowly, then the other train might appear to be moving just somewhat fast, but still faster than it really is.
(All of these cases really come down to “the difference of their velocities”, of course, or the magnitude or absolute value thereof, which is in some sense the “relative speed”.)
Also, especially in the “moving quite fast in the opposite direction” case, when the other train’s beginning or end (front or back) moves by the window that you are sitting by (or the car that you are sitting in, generally), a thing happens with the air between the trains, where a high or a low pressure bulge goes rapidly by your train on that side, and there’s this sort of whooomp sound, which is fun. (It occurs to me that the high-pressure and low-pressure sounds are probably different; I will have to gather more data.) Or not just a sound, really, but a sound and a feeling, or maybe a sound with very significant very-low-frequency components (which is about the same thing).
So there is that!
Addendum to our observations on the S and the 7 the other day: another feature of taking the 7 from Times Square to Grand Central is that if you like miss a luck roll, you end up somewhere underground in a maze of twisty little passages, all but one of which just lead to other subway platforms rather than up into the rest of Grand Central where you actually want to be,
On the other hand the maze also contains a smiling redheaded woman playing the Irish Fiddle in the corner of one passage, which is compensation. :)