Spanish State Railways series 400 and Krauss-Maffei ML 4000 B'B'

For its line between Madrid, Zaragoza and Móra la Nova with inclines of up to 1.8 percent, RENFE ordered a powerful diesel locomotive in 1964 in Germany. It was developed by Krauss-Maffei and based on the Bundesbahn V 200.1. So it had two fast-running Maybach engines with hydraulic transmission, but to match the required train loads each engine had 2,000 instead of 1,350 hp. Upgraded tracks allowed the locomotive to be still carried on four axles with an axle load of 22 tonnes. With its total power of 4,000 hp, this model was called ML 4000 B'B' by its manufacturer.

A special requirement for such a high-powered diesel locomotive was the suitability for the climate in Spain. Here it had to cope with up to 45°C and very high humidity in the summer, but also cold winters in the mountains with up to -20°C. To create enough space for the cooling system, the body was lengthened by almost two meters.

The first ten locomotives were built by Krauss-Maffei in Germany. 22 more were built under license by Babcock & Wilcox at Bilbao. On the ramps, they hauled passenger trains of 800 tonnes and freight trains of 1,000 tonnes. Compared to diesel-electric locomotives with bigger, slower-running engines, they needed more maintenance. So they were already retired by 1987 and two are being preserved on static display.