The most important line of the Norte were the 400 miles from Madrid towards the Atlantic coast to Hendaya in France. Previously, different locomotives were needed for the long straights and the inclines. In order to be able to save time and money on changing locomotives in the future, an express locomotive was specified that had to be able to haul heavy express trains in the mountains. The requirements specified a 4-8-2 wheel arrangement, also known as “Mountain”, and also determined a four-cylinder compound engine of the De Glehn type. With a train weighing 400 tonnes, 55 km/h was to be achieved at 1.35 percent and 90 km/h at 0.5 percent. In addition, due to the ash-containing coal to be used, a large grate was required and the axle load could not be higher than 16 tonnes.