Queen Mary will do bridge tours sometimes. Engine room tours are very rare (I have seen a few).
The gas turbines on QM2 are mounted up by the smokestack, andway, and they have pods for engines.
Ships that have pods do not have the same sort of interesting engine rooms as the propellar driven ships have, and most new ships have pods. On the propellar ships you could see the the screw (the shaft the propellar mounted on) being turned by the electric motors, but not so with the pods.
I haven't seen one, but I doubt there is as much to see anymore as there used to be since pods came along.
By the way - go to the Queen Mary 1 in Long Beach, CA - you can see evey aspect of her engine room, and it is fascinating.