Save a little money and go with the Amtrak provided liquid soap/cleanser; it cleans nicely and smells great!
If you can get your hands on a Amtrak "personal amenities kit"... I would use the stuff inside too.
They're little velvet bags that contain Shampoo, Conditioner, Lotion, Soap and a Shower cap. It's all high-quality stuff from Gilchrist & Soames and I agree with NE933... it cleans nicely and smells great. As a matter of fact, I liked it so much I brought the leftovers with me and used it in my hotel instead of the crap they provided.
But Amtrak's policy on these kits is frustratingly inconstant.
On the "enhanced" routes (CS & EB) the SCA's leave a kit for you in your room when you board.
On the other routes there is just a tub of soap bars in the shower room... but the SCA's should have some of the amenities kits available by request.