These semi trailers are highly specialist and rarely become available. Suited best to high volume, low payload trunking operations, these trailers are usually kept on a fleet for many years. The overheight trailer will contain two levels, often with an internal tail lift to reach the upper level. Loading these trailers is therefore more difficult than a standard trailer and additional training is required. The trailers can be either curtainsided or a box style trailer which can be loaded only from the rear.