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If we talk about Johann #05:
Please make sure that you are dispatching a complete set to this contractor. Your locomotive has to pull maximum amount of its set wagons in order for the SET to be complete.
(No Gem cars needed!)
Chrome Trainset:
https://trainstation.wikia.com/wiki/Chrome_Cargo_I
Please make sure that it is displayed as a set:

Pay attention to the SET symbol in the yard and trains manager, this will show you if a SET is complete.
more Info → http://trainstation.wikia.com/wiki/Category:Train_Sets