D5705 is BoCo's brother, who was only mentioned in an annual. In another annual, BoCo claimed to have a twin called "CoBo" in order to play a trick on Bill and Ben. D5705 is in fact a real Diesel engine preserved and under restoration at the East Lancashire Railway.
D5705 survived by historical accident as a technical services engine and then carriage heating unit TDB968006 before being preserved in 1980. The Class 15 Preservation Society has signed an agreement with the owners of D5705 to become its custodians during its restoration and operation for the next ten years, although funding will remain separate.
D5705 is painted in the green livery of British Railways with yellow warning panels.
- 1987 - The Island of Sodor: Its People, History and Railways (not named, mentioned)
- 2005 - Sodor: Reading Between the Lines (mentioned)
- 1999 - Bill and Ben (mentioned)