You have already paid all your tips when you paid your fare. If you have a regular seat in a regular bar with regular bar staff you may want to leave them something extra at the end.
No. You don't get a receipt to sign (usually) they just swipe the card on their tablets.
Tipping is definitely not a P&O (or British) thing. Tipping to "assist with service" is seen as bribery by many, and will be frowned upon if you are seen to jump the queue by offering an incentive for preferential treatment.