Rebecca is based on the Southern Railway "West Country" and "Battle of Britain" class (also known as "Light Pacifics"), a class of 4-6-2 tender locomotives designed by Oliver Bulleid and built between 1945 and 1951.
While Rebecca is modelled on the unrebuilt version of the locomotive, her tender design resembles the cut-down version used by rebuilt and unrebuilt members of the class in British Railways service. Additionally, she features several differences over her real-life basis, including an altered front end with a larger and lower bufferbeam, driving wheels which share the same design as Hiro's, and plain disc wheels for her front bogie, trailing truck and tender, as opposed to the Bulleid Firth Brown disc wheels found on her prototype. Rebecca's cylinders and cab roof also have a significantly different shape compared to those on the real "Light Pacifics".
Rebecca is painted yellow and navy with white lining. Her wheel rims are painted white, her front end is painted red and she has red and orange stripes painted along her sides. Rebecca also carries two blue nameplates on her smoke deflectors, each with her name in white lettering. Her cabsides have the number “22”, and her tender has the letters “N.W.R.”, both painted in white.