My wife and I took the Celebrity version of this tour a couple years ago. I assume Carnival offers the identical tour. We really enjoyed the trip (we had the "dome car" version), and can highly recommend it.
The tour is about 5 hours long and is about 1/2 train time, and 1/2 bus tour time at the Pacific end of the Canal. The train is very nice, and is high-speed (up to 70mph if I remember correctly). It parallels the Canal and you will see it briefly several times. If you are primarily interested in seeing the entire Canal though, this might not be the best choice.
At the Pacific end of the train trip you will board buses that will take you to Mira Flores Locks and a tour out to the Pacific Ocean where you will see the Bridge of the Americas, some former U.S. Bases, Panama City in the distance, and the Canal Headquarters. After completing the bus tour, you will be returned to the train station for the trip back to Colon.
I have been on 12 cruises and this tour ranks as one of the best of any tour I have taken. If you decide to go, book early because it sells out.