The P class were rebuilt from 1950s T class locomotives by Clyde Engineering. The rebuild involved a new cab and carbody, replacing the EMD 8-567C engine with an EMD 8-645E, replacement of the main generator and traction motors, and provision of a separate head end power generator.
Only first-series T Class locomotives were suitable for upgrading because their frames were designed for EMD 12-567 engines, with only 2/3rds of the length used to cut down on weight (and allow a Bo-Bo, instead of A1A-A1A or Co-Co wheel arrangement). When rebuilt, the 8-645 engine took the place formerly occupied by the 8-567, and the remaining space was utilised for the head-end power unit.
The first eight of the class are currently all in service with V/Line; Pacific National has one engine stored, and Ettamogah Rail Hub has four. In passenger usage are most commonly used in push-pull mode with H type carriages, but also operate as single locomotives if required. In freight service the head end power was occasionally used for the powering of refrigerated containers.
When first introduced, it was hoped to allow push-pull operations with one locomotive permanently allocated to a single three- or four-car H set, with a driving cab installed in the non-locomotive end of that set. This failed because while the Harris motor cabs were sufficient for 1950's standards of comfort and visibility, the conversion cost any application of the grandfather clause and so visibility of signals etc. was deemed insufficient.
Bendigo Rail Models, selling through Auscision, released a model of the P Class locomotive in 2014.
Models in V/Line Orange (P11, P14, P16 and P18) and both Freight Australia (P19 and P20) and Freight Australia with Pacific National decals (P21 and P22) are fitted with the original head-end power fan arrangement, handrails and small side mirrors. Models in V/Line Passenger Mark 1 (P11 and P15) are fitted with the modified head-end-power fan arrangement and the large side mirrors, but the original handrail layout. The remaining models, V/Line Passenger Mk2 (P13 and P17) and Mk3 (P11 and P12) are fitted with the modified head-end-power fan arrangement, the large side mirrors, and the modern handrail layout. The range does not include any models of locomotive P23.