My tipping for the SCA depends totally on what service I received. This summer we had one SCA who helped me with even my carry on bag, brought water around frequently, asked when we would like our bed made down and had the room looking great when we got back from Breakfast. He kept the bathrooms clean, the shower stocked, though he did ask that if any passenger saw a mess to please let him know. He was generally in the car except during meal time when he was getting meals for others. He would stop by to say Hi asking what he could do to make the trip more enjoyable. He I tipped $50 for both of us. He also took everyone's bags to the door downstairs when getting off.
In the Dining Car, the server who remember our preferences, made suggestions, smiled, and generally took good care of us, she got a lot more than my normal ( $2,$3, $5.)
Yes, the Amtrak staff receives a good hourly wage, but gratuity is known to be a part of what they will earn when they are hired. Giving a SCA a big tip, I hope is rewarding and incentive to continue the good work. The MIA SCA gets a zero tip and a letter to Amtrak. The good SCA also gets a praise letter to Amtrak. Many of these Amtrak employees take pride in their work, making passengers happy, and are long term in their positions. This trip I met two SCA's who had 15 years on the same train as an SCA. They always smiled, dressed smartly, and were there for the passengers.