FWIW....
Did both the Belmond Eastern & Oriental (Singapore to Bangkok) and the El TransCantábrico' Gran Lujo (Spanish Basque region) trains last year. Neither journey required tipping (all inclusive fare). However, if your cabin attendant is attentive (as our E&O butler was), I recommend $5-$10USD per day.
Please come back and post a trip report when you complete your journey; this trip is on my list (next 1-3 years).
FozzieDoggie