Got this little gem for Christmas and played last night. I understand it except I think I'm missing something.
You get a 3 cart for a 3 route. Then you get a 4 cart for a 4 route, right? Okay, then I have a 3 cart and I completed a 5 route. Do I get the 5 cart or the four? Is it sequencial(3-4-5 etc.) or does it just depend the size of the route(3 cart, 6 card route so I get a 6 cart).
Just for the record, we played it that you had to have a 3 cart to get a 4 cart. 4 cart to get a 5 cart and so on.
Which is the right way?
You played it correctly. You cannot acquire a cart until you already have the cart of the next smaller size.
In your first example, if you have the 3 cart and you make a 5 route, you earn the 4 cart for this. This is still something you might do if the 5 route was helpful in terms of getting the houses you want down, but it does not get you the 5 cart.