Music/Musicians on the Tube

Not sure where this goes so I try transport...

Are underground musicians common or not on the Tube or Trains?  Or even allowed legally?
They can be such a nice spectacle in NYC, or annoying if you're not in the mood!
I dont mean inside the trains/tube, but at the stations/platforms.

(I just mentioned it because although it's been a while, we had so much fun and such a party while waiting for the subway last night. It's so cool how one unassuming musician can move so many people.  People were dancing all over the place, until the subway came to sweep us away... {it can be a long wait at odd times, although it's very nice to have the trains run 24hrs. I hear it's not like that everywhere?})

Then of course, there's all the other spectacles you'll see at the Times Square stations...
Some with permit, some without...

London doesn't have musicians (or beggars* or lunatics telling everyone about the end of the world) on the actual trains, but there are musicians in some of the stations.  Transport for London (TFL) actually has a busking program for musicians:

http://www.tfl.gov.uk/corporate/projectsandschemes/2435.aspx

*To be fair, I've seen a couple of beggars on London tube trains, but only 1 or 2 over the last 2 years or so.

I see beggers very rarely on the tube, buskers more often.  It is illegal and tfl don't like it.  I find musicians on the train annoying and intrusive, but I love the official buskers to which camoscato refers.

Vicky