The world's first transcontinental railroad, the Panama Railway, was completed in 1855 across the Isthmus from Aspinwall/Colón to Panama City. In April, 1856 a conflict known as the Watermelon Riot occurred when a mob of angry Panamanians attacked a group of American travelers who used the Panama railroad to travel to and from the California gold rush. The outbreak prompted the US to send troops in September, 1856 to protect the railroad stations. This was the first of many times that the US used troops to intervene in Panama.