The 1930s were a decade of innovation and modernisation for DSB. New railway bridges were built across the Little Belt (1935), the Storstrøm (1937) and Oddesund (1938), eliminating the costly and time-consuming process of transfer by steam ferry. The suburban lines in and around Copenhagen were electrified for multiple-unit operation at 1,500 Volts DC ("S-trains"). Early experiments with Diesel propulsion led to the development of the all-purpose MO class heavy diesel-electric railcar equipped for multiple-unit operation, after World War II also fitted for push-pull operation with a driving trailer. Several classes of mainline diesel-electric locomotives were also built as prototypes by Burmeister and Wain of Copenhagen and Frichs of Aarhus, but further development was cut short by the German occupation and the consequent shortage of oil supplies, forcing DSB to rely on coal-burning steam locomotives for mainline duties.