Guess it depends on the metro system.anything can happen in the metro....
I don't think you'll see such a thing on the London Underground! Especially in peak hours! (then the trains are almost litterally overcrowded!)
But in Brussels, you can see the same things. (and no, in Charleroi not! We do have a metro system, but the tram driver can see you literally! And it is a light metro system. (Métro léger de Charleroi, google it)).