I've brought my small (but not backpacking) tent in my luggage many times no problem. I have a large size LL Bean roller duffel and all my camping equipment (including tent, Thermarest pad, light-weight sleeping bag, hiking boots, cooking gear, etc.) fits inside it easily. It's well under the size and weight limits and fits in the over-the-corridor space in a Viewliner-I roomette.
One time on the EB, my SCA put it in some external cubby hole, or possibly in the baggage car for me when I told her I wouldn't need it until I got to Seattle. She went outside with it (so didn't drag it through the other sleepers to the baggage car), and was only gone for a few seconds. Worrisomely, the train stopped about a mile north of CUS because something wasn't secured and I had paranoia that my duffel had fallen out, but there is was on the luggage cart when we got to Seattle.) I've noticed there are some doors above the trucks on the lower level of Superliner sleepers... Does anyone know if there is additional stowage in there or are those doors just for some sort of equipment access? I thought at the time that was where she put my camping bag, but I didn't actually see her do it.
The only think I don't pack is fuel for my stove, which I think is illegal. I buy some on arrival and if I have any left, there are always people who can use it at virtually every place I've ever gone camping, so it never goes to waste.