Why Does a Bullet Fired from a Bullet Train Appear to Travel at 0 MPH?

[bondinthesand]

So today in physics my teacher was talking about the bullet train that is in japan. SO this train is traveling at 190 miles an hour, and on this day there is no gravity and no air. a man walks to the back of the train and fires a bullet from the back. My teacher said that a person watching the train go by will see the bullet go 0 mph. why is that??

 

[S_Happens]

If you fill in the details, then you can start to create a picture for yourself.

First for the example your teacher would have to state that this bullet travels at 190 mph, which is AWFULLY slow (may a low power air rifle). Given that information what is the only way to get 0 mph from two objects, each traveling 190 mph separately?