The long Trans-Iranian Railway from Bandar Torkaman on the Caspian Sea to Bandar Shahpur on the Persian Gulf was opened during the reign of Reza Shah Pahlavi in 1939. The railroad was built with rail weighing 67 pounds per yard (33 kg/m) and required more than 3000 bridges. There were 126 tunnels in the Zagros mountains. The longest was 1.5 miles. Grades averaged 1.5 percent south of Tehran, but then increased to 2.8 percent to cross the 7,270-foot pass between Tehran and the Caspian Sea. After the Anglo-Soviet invasion of Iran in 1941, this Persian Corridor became one of the supply routes for war material for the Soviet Union during World War II, (Railway trend in Iran). The British built a branch line from the 3000-foot (900-meter) bridge over the Karun River in Ahwaz to a new southern port at Khorramshahr on the Shatt al-Arab river. In 1943 3,473 American soldiers of the Military Railway Service began running trains between the Persian Gulf and Caspian Sea using ALCO RS-1 locomotives rebuilt with 3-axle trucks and designated RSD-1. The Americans set up headquarters in Ahwaz, but were unable to tolerate the daytime heat, and generally operated the railway at night.