I have never much paid attention to what order my collectibles are in but now i am looking at buying the train pieces so it kind of matters. i want to make sure i buy them in the right order so they line up correctly. would i start with the engine and go in in order to the caboose or do i start with the caboose and buy in reverse order? also do i need to purchase them one at a time or can i buy them all at once so long as they are in my cart in the right order? thanks for the help