Most expierenced riders would disagree with this "tip". Tips are to show appreciation for good service rendered, so if you get one of those invisible slackers that do show up on LDTrains, you've wasted your $20 tip( aka bribe).
So tip your SCA at the end of each leg of your trip according to the service you get, just like the servers in the Diner and any Redcaps you might use during your journey..