Luca is an LNER A3, (formerly a LNER Thompson A1/A1)
Luca is based on the LNER's A1/A3 Class of 4-6-2 Pacifics. Designed by Sir Nigel Gresley for express passenger work on the Great Northern Railway (a predecessor to the LNER prior to the Big 4 Grouping of 1923), and were based off of the Pennsylvania Railroad's K4 Class of 4-6-2 Pacifics (Though they lacked the Belpaire Firebox, instead having a more traditional roundtop one). A total of 79 locomotives (52 A1's and 27 A3's) were constructed, with the majority constructed at the LNER's works in Doncaster, though 20 of them were built at the North British Locomotive Co. in Glasgow, Scotland. Almost all of the A1's were rebuilt to A3 Specifications between 1928 and 1949. The Class lasted until 1966, when the last member, 60052, "Prince Palatine" was withdrawn by BR. Only one LNER Pacific survived into preservation, #4472, "Flying Scotsman", who was the very first to officially travel 100 MPH.
Like most of the Thomas Characters, Luca has a few differences compared to his basis, including a larger boiler, the addition of a Fowler tender, and the use of either ovular or square buffers as opposed to circular buffers. Gordon is also another member of this class.
Thompson drew up two modified 3-cylinder designs both of which discarded the Gresley conjugated valve gear and had separate sets of Walschaerts valve gear for each cylinder. Due to space restrictions, the middle cylinder could not be parallel with the outside cylinders. One design had the middle cylinder forward of the outside cylinders, whilst the chosen design had the middle cylinder further back. Both designs used A4 boilers. A new set of frames was built for the rebuild. These were similar to A4 frames except for the wheel spacings at the front. Thus very little of the original locomotive was retained. The resulting wheelbase was 38 ft 5 in (11.709 m), even longer than the Gresley P2s. Luca suffered Mechanical Faults from this!