During Taylor Swift's "Red Tour" stop in Newark, New Jersey (she played three nights, by the way...), the mega-star was joined by a very special guest on the last night.

Train's Pat Monahan!

Swift told the crowd, "I called up one of my favorite lead singers of one of my favorite bands of all time and asked him if he'd like to come play with me in Newark.  And you know what?  He said yes!"

And here is the Swift/Monahan duet -- Train's "Drive By."