How long will it take a mile long train going 15 miles per hour to get completely through a mile long tunnel?

Depends on what "out of" means.

At 60 miles per hour (one mile per minute), a train entering a mile-long tunnel will first emerge one minute later.

But for the full one-mile length of the train to pass through the tunnel and exit will take another minute, totaling two minutes from the time the first car enters to the time the last car leaves.