You can have water lush toilets without having retention tanks.
toilet types:
1) dry hoppers dropping to ground
2) flush toilets dropping to ground
3) flush toilets to retention tanks (and early example are the Am-fleet cars)
4) flush toilets that go to retention tanks when train is moving below a certain speed; then dump to ground above that speed (early Amtrak Super Liners did this).
So, the question remains when did the Silverton change over to retention tanks?
BN
PS most passenger cars in service on the "mainline" roads prior to Amtrak had flush toilets dropping to ground. They also carried signs saying not to flush toilet while train was stopped at stations or when it was at a crossing.