I don't have the Asia pack yet, but 6 player options are pretty appealing to me.
Some of the official maps are designed for fewer players, so 6 wouldn't be a great idea.
You could maybe do USA, but you would have to limit the number of trains - the Asia map is just plain bigger.
Yes you get those nice wooden card holders with the Asia expansion.
The thing is that the normal maps with 5 player support demand 45 trains per player.
The team Asia says 48 for each of the 3 teams which is 24 trains per player. So 225 trains vs 144 going down if everyone uses all their trains. I think I need to count up all the rail links on the boards and suggest a reasonably scaled, even number of trains.