In Moscow for example the underground is called metro. The first metro line was built in 1935 but most of the stations were built during 1950's and 60's but new ones were constantly built since then. Nowadays it has 185 stations and 305,7km of rails. The escalators at some stations are very long and steep. An interesting fact, I figured out just now? If you move towards the city center, the voice which announces the stations is male, and if you move from the center, the voice is female. I never paid attention to it!
Moscow's metro is one of the mainly used transportation methods for the citizens and it has the important task to transport almost 7 million passengers per day to any spot of the capital. I hope you can imagine how crowded it is at rush hour! So if you are claustrophobic, don't use the metro by any means. (Well, if you are claustrophobic, don't live in Moscow at all! It is the largest city in Europe, so what do you expect?)
The metro in such a large city like Moscow does not run 24h per day, as you may have thought. It closes down at night, as it does the Tokyo underground. So if you get stuck in the city in the center of the night you have very much of a problem. (Yes, that happened to me and my friend once and we almost got shot. I don't want to dramatize, as I am still living and wasn't hurt but don't try this out kids, really.)
By the way, the metro of Moscow is very safe and with many precautionary measures in cases of emergency (like hermetic doors at some stations). People can even hide there in case of bombings. Also, the stations are very clean. I've never seen any extreme cases of vandalism or dirty bums sitting there.
But everything written above are just boring facts. Let's move to the really tasty stuff: The metro of Moscow is the most beautiful metro of the world! Indeed, the palace of Versailles looks like a baggers hut in comparison to the glory of Moscow's stations. (And I am not exaggerating! Ok, maybe a tiny little bit…)
The stations are marble-lined, with colourful mosaics and and sculptures. You can walk through and feel like a king in the crowd. You can judge by yourself, if you look at the first picture, shameless stolen from Wikipedia). Why have people put in the effort to build it like like that? I heard, it was an order from Stalin himself, who wanted to show the power and beauty of mighty Russia.
Let's move on to the subway of New-York! To cut a long story short: New-Yorks subway is ugly. And filthy. I can clearly recall seeing a fat rat down there, crossing the rails. In addition, the ventilation is horrible! I was there in summer, like tourists usually do, and I thought I will choke and die down there! It was hot like in an oven and airless like in open space. I can imagine, in winter it's the opposite - bone-chilling cold. The stations are painted black inside (as if it is not dark enough under the earth) and one can see ugly cables and metal constructions. (See third photo)
The good thing? In New-York the subway is 24/7! (Whether it's very safe to use it in the middle of the night is a different matter).
Some of the subway lines do not under the earth, but on steam punk-style resembling metal bridges, which look, like they can collapse every second,above the roads.
To give you some interesting facts? It was built mainly in the 1940's, transports 5 million people per day, and owns 337km of rails. A very interesting fact I heard is, that the light bulbs at the stations are inserted the other was round, so people don't steel them for their own needs (the ceiling is very low, so maybe if you use small ladder, you could reach it without problems).
Ok, last one is Berlins U-Bahn. I cannot say much about it, because I rarely visit Berlin. Again, it is not very spectacular. It is clean for the most part, but you can see on the last photo, it looks very boring. It transports just 1,3 million people per day and has 173 stations. Interesting is, that according to noobs-friendly-encyclopedy Wikipedia it was opened in 1902! Almost 40 years earlier, than the other undergrounds. Also very interesting: If you enter the U-Bahn (or was it the S-Bahn I used, which partly runs on the underground rails?) you don't have to go through tourniquets. There is just a ticket validation machine at the entry. So maybe sometimes there are controllers going trough and checking whether you have a ticket (I don't know, I've never been controlled lucky me… as if I would ever be a fare dodger, ha ha… *nervous giggling*) So it seems it mostly relies on the honesty of the Germans.
I think I finish off my post at this point. Thank you in advance for reading it, I hope it is interesting.