Your ears "popping" happens when there is a big change in air pressure. It either presses on your eardrums or "sucks" at them, making them bulge out. Either of these is uncomfortable, even painful; it interferes with your hearing and could cause harm. To adjust to the changes it is necessary to let air from outside into the middle ear, the space behind the eardrum, or let out air from the middle ear. The way we do that is to open a channel, the Eustachian tube, from each middle ear to inside your pharynx, the passage between the back of your throat and the back of your nose. That causes the popping as the air rushes in or out. Then the tubes shut tight again, to keep muck out of the middle ears.
When a train moves through the air, it pushes the air in front forward, but the air soon pushes out sideways (or trains could not move much at all). When the train rushes into the tunnel (a slowly moving train causes no ear popping) the walls of the tunnel get in the way of the air that would be rushing backwards past the train. It now has a far smaller space to get past, so it pushes very hard; this is the increased pressure that makes your ears pop. They might pop back again when you exit the tunnel, but that does not always happen.
As for the mirror effect, whenever you through a glass window you may not realise it, but you always see some light from behind the window, and (usually less) light reflected back from your side of the window. When the outside light is much brighter, you don't notice the reflections much. At night with your lights on you can use your windows as a fairly useful mirror. (In the old days good mirrors and glass were rare, hence the remark: "For now we see through a glass, darkly...") That is when there is too little light from outside to drown out the reflection from inside.
Where I live wild guinea fowl sometimes pass our windows and, there being more light from outside than inside, they see their own reflections and pick fights with them, pecking at the glass. To prevent this, we draw the white blinds behind the glass. This reflects the light from outside and drowns out the reflection, giving peace to us and our cat, who otherwise is frustrated by not being able to get at the insolent birds. Pin-tailed whydah males are even worse; they are small birds, but want to boss everything in sight, especially their own reflections!
You might in the UK have similar problems with tits or sparrows for example.
Well anyway, when you are inside the tunnel there is usually light inside the train and not much to illuminate the filthy tunnel walls, so you see your own reflection more clearly, and a good deal better to look at than what is outside usually!
