In 1918, the historical regions of Transylvania, Banat, Bassarabia and Bucovina united with the Kingdom of Romania to form Greater Romania. As a result, all railway lines in these regions, previously under the ownership of Austria-Hungary or the Russian Empire, were placed under the administration of CFR. This event was significant for Romanian railways because it meant that the large Arad rolling stock and steel factory, previously located on Austro-Hungarian territory, was now part of Romania, and was consequently used to produce a wide range of rolling stock and locomotives for CFR. During this time, various railways were "doubled"— that is, double tracks were introduced on routes to permit a greater flow of traffic. The first line to be doubled was the Bucharest–Ploiești–Câmpina line, where doubling was completed in 1912. In the period between the wars, various other lines were doubled, including:   During the 1920s and 1930s CFR also bought several new locomotives and cars, embarking on a plan of widespread modernisation. Diesel traction was introduced, and the Malaxa high-speed DMUs (a modernised version of which are still in use as commuter trains in Banat) were built. A significant event in the history of the railways was the Grivița Strike of 1933, which occurred during the Great Depression and resulted in violent clashes between railway workers and police. CFR (Romanian Railways) was involved in the transport of Jewish and Romani people to concentration camps in Romanian Old Kingdom, Bessarabia, northern Bukovina, and Transnistria (Romania). In a notable example, after the Iasi pogrom events, Jews were forcibly loaded onto freight cars with planks hammered in place over the windows and traveled for seven (7) days in unimaginable conditions. Many died and were gravely affected by lack of air, blistering heat, lack of water, food or medical attention. These veritable "death trains" arrived to their destinations Podu Iloaiei and Călăraşi with only one-fifth of their passengers alive. No official apology was released yet by CFR for their role in the Holocaust in Romania. After Romania became a Communist republic in 1947, there was quite a significant amount of investment in railway infrastructure, often to the detriment of other forms of infrastructure. During this period, railways were seen as the symbol of Romania's rapid industrialisation under Communist leaders Gheorghe Gheorghiu-Dej and later by Nicolae Ceaușescu.