[6] In this series of locomotives, instead of taking the typical single cabin General Motors design it was decided to use a two cabined design; the Scandinavian manufacturer NOHAB had also been producing GM engined locomotives (having acquired a license to produce GM engined locomotives of its own designs[7]), and a model was chosen based on the JT-26 with the Nohab bi-cabin box (See DSB class MZ) which was used by the Danske Statsbaner (DSB).

[6] Renfe chose the variant model MZ1 which had a power output of almost 4,000 hp (3,000 kW), they were also able to supply electrical train heating - a first in Spain.

The first units were assigned to Madrid Atocha depot, where they progressively replaced class 340 on the Madrid-Burgos railway and later in other lines.

Despite so, they were outran in 13 minutes by well-maintained class 340 on the 329 mile stretch between Madrid and Móra la Nova.

Ourense-junction to Medina del Campo route was served by the class 333s, which also were found in applications such as transporting coal from Samper de Calanda to thermal power stations in Andorra, and working in pairs to bring ore from Ojos Negros to the steel works at Sagunto.

The rebuild gives improvement of 8–10% in tractive effort over the original machines, which is explained by the use of the power electronics system.