In search of more powerful diesel locomotives, the Denver & Rio Grande Western ordered three ML 4000 from Krauss-Maffei in Germany in 1961. With hydraulic power transmission and two high-speed Maybach engines with 2,000 hp each, their basic technical concept was completely different than that of American locomotives. The advantages lay in the better power-to-weight ratio and better traction properties of the three-stage transmission. In order to cover the development costs, Southern Pacific was persuaded to order three more ML 4000.
The operators' network included passes in the high mountains over which trains weighing more than 10,000 tons had to be hauled. The problem turned out to be that the engines were designed for a full throttle share of ten percent, what matched German conditions. In the USA, however, this proportion is many times higher, which, in conjunction with the sometimes hot climate, led to problems. In addition, maintenance was found to be too complex compared to the much simpler American locomotives.
The problems were initially so serious only at the D&RGW that its three locomotives were sold to the SP, while the SP ordered a second batch of 15 locomotives. While the first batch had a self-supporting car body, the second batch was built in the same way as American road switchers with a load-carrying frame and machinery covered by hoods. At the same time, the Brazilian Estrada de Ferro Vitória a Minas (EFVM) ordered 16 ML 4000 in meter gauge.
Although the SP was initially happy with the ML 4000, problems with the air intake in the long tunnels of the Sierra Nevada soon became known. For the same reason, EMD developed some locomotives with modified airflow, which became known as “Tunnel Motors”. When the maintenance of the engines became too expensive, the locomotives were retired by 1968 with the conclusion that the hydraulic power transmission itself was reliable and economical. The 9010, later numbered 9113, was converted into a camera car in 1969 and was donated to the California State Railroad Museum in 1986. There it was refurbished and in 2017, it made its first run with its rear engine, while the front one is still out of order. The Brazilian machines, on the other hand, remained in use until the 1980s.
