During the first world war, the cost for coal increased rapidly, and also the electrical operation was affected by this as the power was generated mainly in the coal fired power plant in Stocksund. To get access to cheaper electricity an agreement was made in 1916 with the hydro power plant in Älvkarleby in the north of Uppland. The deal meant that cheap "second grade" power (that could be cut off by short notice) was bought at very low cost. An AC/DC convertor (motor + generator) was installed in the power plant in Stocksund to convert the high voltage AC power to the appropriate DC voltage for the trains.
In the remaining years of WW1 the electrical
operation really proved its point as the cost for electricity kept going
down, while the coal prices still were rising. By 1919 the roster for the
electrical lines had grown to 16 motor units and 13 trailers (of which
three was open air "summer carriages"). They operated 17,9 km of electrical
lines with a seating capacity of 1236 seats (not counting the "summer carriages")
and "produced" 19,4 million "personkm". In the same year the "steam lines"
had a seating capacity of 2845 seats (2,3 times as many) and produced 31,9
million "personkm" (only 1,6 times as much) on a network 11 times as large
as the electrical network!
The first attempt to get rid of the steam
engines on the non electrified lines was made in 1924, when Deutsche Werke,
Kiel (DWK) delivered motorcar 101. Initially fueled by spirits it was later
converted to petrol. Experiences were not overwhelmingly positive, and
101 eventually was rebuilt into a passenger car in 1941. The next attempts
were made in 1935, when Nohab delivered number 102 and 103. These were
followed by several more units in 1938 and onwards.
SRJ had been operating a standard gauge trolley line between Stocksund and Långängstorp since it was opened in 1911. This line was regauged from 1435 mm to 891 mm in 1934! This year two new motor units was added to the roster.
In 1937 the last major extension was completed, when the line from Djursholms Ösby to Näsbypark was opened. The first part to Altorp was opened including electrical operation already in 1910. The second part to Lahäll was opened in 1928, also with electrical operation. This line was so close to the most northern part of the former Djursholmsbanan, that the later was closed and abandoned between Eddavägen and Svalnäs in 1934.
Next the plans were to extend the electric operation to Österskär and Vallentuna. However, with the additional line length it would be hard or even impossible to maintain a voltage of only 660 V due to the additional losses. The alternative would have been to build very many feeding points with local converters along the line. At the same time it would be very costly to convert the entire network including existing electric motive power to a higher voltage.
The solution was a compromise worked out by ASEA. The new lines would use 1500 V DC, while the existing network was converted to half the voltage, 750 V. This way the existing rolling stock could be used on the older part of the system with minimal changes, while the new rolling stock would work with both 750 V on the old part of the network, and then switch to 1500 V operation on the newly converted lines.
On May the 15th 1939 the new 1500 V system was opened for operation to Vallentuna and Österskär. The switch over between 750 V and 1500 V was made in Enebyberg between Djursholms Ösby and Roslags Näsby. The original single overhead wire was replaced with carrier wire and main wire to allow better and more even pick up and increased speed up to 75 km/h. Eight new motor units - X4p - was delivered in 1938 with one following in 1940.
The fact that the major part of the commuter traffic at SRJ during W.W.II was made with electrical trains, saved a lot of money for SRJ. In 1941 alone, the saving was 333 000 Swedish Crowns, 15 % of the total investment for the electrical operation to Österskär and Vallentuna.
The next extension of the electrical operation was opened in 1946, when the entire main line up to Rimbo was finished. Finally in 1949 the line to Norrtälje was also included in the electrical operation. The electrical roster had by now been extended with five electric locomotives built by ASEA in 1942 (2) and 1946 (2) and one by Siemens in 1947. These were mainly used in the freight trains on the main line. Eleven new motor units (X7p) had been delivered in 1946 (8) and 1949 (3).
With the deliveries in 1946, it became
possible to upgrade also the lines close to Stockholm to 1500 V operation.
Six of the older motor units were rebuilt from 750V to 1500V, while the
others were rebuilt to trailers.