Tipping isn't required or even suggested by Amtrak so I don't do it. Crusie ships suggest it, so I do. If the company doesn't say it's proper I figure they know best. I do always say thank you, for good service no matter what.
So does that mean that you don't tip in restaurants either? They don't suggest it or require it either, unless maybe you're with a party of 8 or more. Most hotels don’t suggest that you tip the bellman or the concierge, yet people do. What about cab drivers? Their companies don't suggest it or require it. And then there are the pizza delivery people or other food deliveries, their companies don't suggest it either, yet most people do tip the delivery person. I've also never seen the US Postal service suggest or require tipping, yet many people give their postman a Christmas bonus, which is still a form of a tip for good service.
Cruise ships suggest it because they know that by doing so they can underpay their employees. They'll tell you that it's to motivate them, and yes that's part of it. But that's not the real reason that they promote the idea. Just like your hotel telling you that by putting your towel back on the rack is the green thing to do. Sure it is a green thing, but their real motivation is saving money. They have to buy less soap, they need less hot water and water in general, and they need less people working in the laundry room. Every time you don't drop that towel on the floor, you're saving that hotel money.
With respect, just because the company didn't tell you to do it or suggest it, doesn't mean that it's right or wrong.