Diesel locomotive break down. Hydraulic transmission for locomotives was developed in Germany. There is still debate over the relative merits of hydraulic vs. electrical transmission systems: advantages claimed for hydraulic systems include lower weight, high reliability, and lower capital cost. By the 21st century, for diesel locomotive traction worldwide the majority of countries used diesel–electric designs, with diesel hydraulic designs not found in use outside Germany and Japan, and some neighbouring states, where it is used in designs for freight work. In Germany and Finland, diesel–hydraulic systems have achieved high reliability in operation. In the UK the diesel–hydraulic principle gained a poor
Electro-diesel locomotive take advantage of shorter sections of electrified main lines. Bombardier has built dual-mode variants of its AGC series for the French operator SNCF; the electricity is collected by means of a pantograph. In Russia, a number of electro-diesels were built which had both pantographs and diesel prime movers; these locomotives were actually multiple units, with distinct electric, diesel, and motor sections. These included: An experimental electro-diesel locomotive, DEL120, was built by London Underground in 1940 but was not a success. Two types have been built whose electricity source was a 750 V DC third rail. Electro-diesel locomotives and multiple units