Cargo bonus engines don't take up room in your engine house or the station concourse. That means you can own one more engine than your competitors. Cargo bonus engines of the current era can only be won in the lottery or purchased with Gold. In very rare competitions there is also the possibility to win cargo bonus engines of the previous era.
Each era has a Cargo bonus engine:
Additionally there are Cargo bonus engines that are available in all eras, which have different stats depending on the era you're in: