Yup,this mission is pretty easy!
Just don't care about Forest Owl's Plan.
When the mission in progress,just follow Rinoa.
When you have to unlock the train,right after you have control of Squall ,go down and enter one code(slowly but SURELY,you must hear the sound of accepting process before you enter next number)then go up and wait until you can see two guards leave the window below you at ONCE, continue to wait until you can't see them(you don't have to use the checking button at all) (the Blue is two times faster then the Red ,so you will have a chance every 30s-40s)now go down and enter two code(don't act too fast,you may miss,just surely is enough),....
I always finish this mission very soon,due to this method.