Board Thread:Questions and Answers/@comment-98.66.220.243-20150416142527/@comment-15636815-20150416224501

Usually, the game wants a player to own both One-Click-Cargo and One-Click-Passenger before allowing other players to send to them trains that contain more than 14 wagons. This is true even if, for example, all the wagons on the train are cargo wagons and the player owns OCC but does not own OCP.

I'm reasonably sure the devs know about this. I'm equally sure they intend to ignore it. Once you buy OCP, you'll be fine.