bag size for coach tour in AK

[framer]

Doing a cruise tour in alaska and will be on both tour coach and train from fairbanks to Denali then coach to the ship. I need a carry on for my photo gear and I think my bag my be too large for the coach. Any suggestion to size for that carry on. Overhead room? under seat room? Princess FAQ's say 17 x 14 x 4 for a coach care. Can it be slightly larger?

 

Thanks

framer

[framer]

I own a old Tamrac 709 16½ W x 10 D x 9½ H. Would that fit under the seat in the motor coach?