Model train layouts are so attractive - to the "trained" and the "untrained" eye - that they are bound to entice touching. It's almost like sculptures in a museum, which are so attractive that visitors frequently want to reach out and touch them. Hence, among the most popular statues, guards are often on duty, just for the purpose of telling people "Please, do not touch. (Keep your greasy, sugar-coated paws off the marble !)" The same even happens in galleries for paintings. People do reach out to touch them, they are so beguiling in their truth.
What's my point? It's a layout's very attractiveness that is causing intrusions to happen - it's almost a compliment to what we have created that folks want to interact with and touch them, esp. when hosting the company of a very, very hands-on-device(s) generation to enjoy our electronic, non-static, miniature worlds.
The advice expressed about asking visitors ahead of time not to touch or, at the very least, to be careful about what they go near, seems a reasonable approach. It is difficult to balance friendliness, an invitation (enticement) into involvement into our hobby, and the practical monetary aspects to these treasures, which takes almost as much skill as crafting the layouts in the first place. IMHO.
(Yes, the several above puns were intended.)
FrankM.
P.S. Concerning my own layout, I have never said a word (it would not occur to me) cautioning or forbidding anybody about touching the layout. For us, Number One is that they are guests in our home, so hospitality is primary for my wife and me. That also means we only invite folks who we anticipate know how to conduct themselves around something so relatively rare and valuable. That has worked out fine for us since the first layout was ready for guests back in 1995. And that includes an entire children's' choir on two occasions.