I spend perhaps 10-15 nights per year in a roomette, both on Superliners and Viewliners. Until perhaps 3-4 years ago, I could always count on one or more bundles of bath towels and individually wrapped bars of soap available in the showers. But that was then, and this is now.
In the past 12 months, I've noticed a general lack of bath sized towels in the shower room...typically not more than 6 (one bundle). Usually, but not always, there's a bundle or two of fresh towels in the luggage rack as well. My thinking these days is that Amtrak is cutting back on towels for the sleepers. Whether it's the result of not many towels being used per trip or just being cheap, I don't know. I've also noticed that on the third morning in the 'through' sleeper (#421/422), it's likely that most of the towels on board have already been used. That's one of the reasons I always try to get in the shower by 6AM or so. If there's any towels to be had, I'll get one. Only once have I not found any bath size towels (I always look before I walk in, lock the door, etc). Knowing that ahead of time, I took both of the hand towels in my roomette along with me to dry off with after using the face cloth I always bring from the roomette to dry off as best as possible.
Soap? I'd say 50/50 on whether there's a bunch of bars of soap (often, still in an 'open top' case, if I could call it that...24 bars total, maybe 36?) In the Viewliners, sometimes there's no bars of soap but there is the 'Amtrak liquid soap' dispenser in the shower itself. I've not seen that in a Superliner, however. So, I usually 'sneak a peek' in the shower shortly after boarding to see if there's soap or not. If not, I have a bar or two in my shave kit for 'just in case'.
I also travel with a couple of hotel-freebie sized bottles of shampoo that I refill at home before I leave. There's no shampoo in the shower...except once, maybe 6-7 years ago on the Capitol Ltd.
I've been surprised to see disposable slippers/flip-flops in the showers in the past 6 months or so. I have a nearly worn out pair of comfortable, fuzzy moccasins that I always pack last. They're on my feet within a minute or two of entering my roomette for the first time. They also come in very handy when a trip to the toilet is necessary, as is a pair of 'thin' sweatpants. I wear both to the shower as well.
Lastly, know that some passengers will leave the shower room in a not-so-tidy condition. Part of this is caused by the big laundry bag for used towels hanging somewhere making the dressing/undressing area somewhat cramped. Most often, one or more early (or late night) shower users will leave the used bar of soap in the shower. I simply grab those and toss 'em into the trash recepticle. Sometimes, soap wrappers are left laying around, they get tossed too. I always position the bath towel I'm going to use to be safely reachable without leaving the shower stall. That way, I dry off completely while still in the shower stall, then take the towel I just used and spread it out on the dressing area floor. There's almost always some water that doesn't stay in the shower while I'm showering. That way, I can get dressed without risking a slip on a wet floor and, as my last step before leaving, wipe the floor dry and put the towel in the bag.