how to keep brother's kids away from trains.

I would tell them that there is a giant monster. Actually, what I would really tell them is that there are very dangerous chemicals in the train room & if they touch anything, they might wish they didn't have. As for the bathroom I'm sure you have found your answer.
 
I was just talking to my brother on the phone and told I was kid proofing the train room and put locks on the door. "I don't think that's nessary." was the answer I got I then said nobody is going in there without me. and silence...

So the locks are in place, they're not staying over night so that's a big relief. I figure once the little bratts are in the livingroom visiting with my mother I quietly go downstair with the cats and lock myself in the train room.
 
Like everybody said before: IT"S YOUR LAYOUT!, YOUR EFFORT, ENERGY, TIME, AND PASSION. You set the rules. If they don't like it, the basement becomes OUT OF BOUND for everyone.

Charlie: Well said