It's the collection of all music played at each station of the Yamanote subway line in Tokyo. (click HERE to view and listen)
By the way, the alert sound before train departs or closes its doors are usually some boring sounds, but in Japan they are usually music.
I don't know if it's real but I've once heard the reason why they started to use music is some foreigners jumped off the train when they used the old scary alarm sound as the alert. So in order to avoid this problem they started to use some softer music (Doesn't sound very realistic to me but this is what I heard).
Anyway, get a feel of a train ride in Tokyo. (^^)b