Last year I took Amtrak from Portland OR to Seattle WA and had to get their bike box. I think it was 20 bucks' total.
The box is huge, you have to remove the pedals and turn the handlebar and roll the bike into the box - they had tapes and eve 15 wrench.
Even big bikes should fit nicely - if the weight is above the limit, then not sure what the outcome will be.
The downside is that I had to wait long to retrieve the bike and had to throw away the box into a garbage bin around.
I had a ticket with carry on bike (5 bucks) that got cancelled and the train I got was all sold out, so I had to check in as one piece of luggage.