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 invading British built a branch line from the bridge over the Karun River in Ahvaz to a new southern port at Khorramshahr on the Arvand Rud 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 Ahvaz, but were unable to tolerate the daytime heat, and generally operated the railway at night. The Persian Gulf Command ran trains day and night.