Well, with the risk of looking like i got no modesty, when it comes to co-op i accept only certain players in my company.That`s because i believe that if i co-op with someone , that person must have at least the same standards of building like me (i wouldn`t want anyone building a rail system with signal no5 or having small curves)
I also support this kind of strategy since co-op works better when both players have the same understanding of the game.
I had games where i didn`t respect this rule...and all the game i was like "don`t touch this, don`t touch that" just because my team-mate started changing them cause of various reasons so i`ve learned my lesson.