These private wagons operated on the national railway network alongside company-owned rolling stock, being run by firms such as Koype or Transfesa, either for their own freight or for third-party transport. RENFE’s rolling stock was divided into three main categories: commercial wagons (owned by RENFE), private wagons or privately-owned stock (where the owner could transport their own goods—such as Koype, Transoil, Elosua, Borges, Saltra, or Savesa—or alternatively, offer transport services to other companies, competing directly with RENFE), and internal fleet wagons (used exclusively for the railway company’s own logistics). Specifically, regarding three-axle tank wagons, innovation came in the 1950s from the factory Herederos de Ramón Múgica, pioneering their design. These wagons increased carrying capacity while staying within axle load limits, making them particularly suitable for secondary lines and industrial sidings. Following their success, many private companies—including Koipe (manufactured by Electrotren in H0 scale and by Arnold in N scale), Transfesa, Elosua (manufactured by Electrotren in H0 scale and by Arnold in N scale), and Borges (manufactured by Electrotren in H0 scale and by Arnold in N scale)—placed additional orders, cementing this design within Spain’s railway freight fleet. The last wagons of this type were delivered to Borges in 1967. Details of models HN6672, HN6673, and HN6674: • Central axle with lateral displacement (minimum radius: 192 mm) • Short coupling mechanism according to NEM 352 on both ends • Coupling pocket according to NEM 355 • Vacuum brake tanks on the end wall without platform • Distance between buffers: 132 mm (66 mm per wagon) • Multiple details made of precision photo-etched metal