I don't think it is rude for them to have a conversation. I have been in Japan a lot too and people talk on the train, shinkansen and local trains. People talk in the elevator in Japan too.

I always travel alone and one time an older lady told a girl one row in front of her to be quiet. This was on a 12 hour all day(no night) flight from YYZ to NRT. I thought the older lady was kind of rude. Who did she think she was? She doesn't own the plane, and the volume at which the people in front of us was not by any stretch of the imagination loud or annoying.

When a baby cries.. that is annoying. But no one ask the baby to stop crying.