Stepney the "Bluebell" Engine
The first one was coloured green and cream and had a yellow face. He/she resembles a BR Class 31 Brush Type 2 A1A-A1A diesel-electric. 31120 is another member of this class. If looked at closely, his/her number seems to be D701. His/her headcode is "4F12".
The second diesel was a maroon colour, and resembles a BR Class 52 "Western" C-C diesel-hydraulic. All locomotives of this class were given two-word names, the first word being "Western" and thus the type became known as Westerns.
The first diesel was painted with yellow and black "hazard" stripes and appeared to be based on a Class 08. Unlike most Class 08 featured in the Railway Series and the Television series, his/her face appeared to be descending from his/her cab, rather than his/her front end.
The second diesel was painted in British Railways' "Rail Blue" livery with large yellow warning panels, and appeared to be based on a BR Class 28. BoCo and D5705 are also members of this class. He/she appears to be D5701, since that engine was the only Class 28 to be painted in "Rail Blue" during the class' existence.
Upon closer inspection of the main line going away from the signalbox, a third diesel engine can be seen in the background. Its class is unknown.
The Passing Diesel
The Passing Diesel is a diesel who shouted a warning that Oliver was escaping. When Douglas was helping Oliver, Isabel and Toad escape from scrap, he/she witnessed the event. Douglas told the stowaways to take no notice of the diesel. The Passing Diesel was not seen in any illustrations, making it unknown what his/her basis was.
James and the Diesel Engines
Several diesels from the Other Railway have visited the Island of Sodor on several different occasions. Visiting diesels are known to be particularly haughty, as they feel superior to steam engines and often put the steam engines down.