When British Rail implemented the TOPS system for managing their operating stock , part of their shipping fleet was incorporated into the system as Class 99 , in order to circumvent some of the restrictions of the application software . When British Rail implemented the TOPS system for managing their operating stock , ships capable of carrying rail vehicles were incorporated into the system as Class 99 . In order to circumvent restrictions of the application software , these ships were entered on TOPS as locomotives , ` hauling ' the trains which they carried on board .