Board Thread:General Discussion/@comment-67.182.231.46-20131226230517/@comment-24392789-20140108210641

Basically, if the train is full and the train builder shows the "set" star-shaped sign, then it's a complete set. The only case where you need to spend gems for a set is when either

- the locomotive costs gems

- you can't buy enough non-gem wagons to fill up the locomotive

So for instance if a loco costs money, and it takes 12 wagons, and you can buy (for money) at most 8 lumber transports of the same set, all the rest of items in the set are sold for gems, then you are forced to buy the pieces with gems. But if it is possible to fill up te locomotive only with normal wagons (of the same set), then the set is complete, no need to use the gem-costing wagons.