I've put my granddaughter in charge of tipping since our first trip when she was 7. Since most of our trips are multi-day coast-to-coast trips there is a pattern. She tips the SCA in the same way her waiter in the diner is tipped--when service is rendered. Each morning before we leave our room to go to breakfast, she leaves a tip for that day on her pillow. (She is usually rewarded with a gift of Amtrak chocolate or a packet of Amtrak postcards on her bed that evening when the SCA does the bed turndown). Upon arriving at our destination, she hands the attendent an additional tip commensurate with the level of service.
I don't see that as a bribe, but as an acknowlegement that ongoing good service has been noticed and appreciated.