Board Thread:Questions and Answers/@comment-29358900-20160924005953/@comment-26692787-20160924023943

Most of them you can sell, but check for each one, if it is buyable in the shop, otherwise you will loose it forever.

Rewarded engines, which will be used in any contracts or achievements are unsellable and take no room in your storage. The other rewarded engines will also have a little use. They can be traded with other players for their museum, be it high level players, who had sold them before the revision of the museum and now miss them, or gifted to new players to help them to advance in museum levels or museum achievements..