He is remembered as "father of the third rail",[1] a pioneer in electrical engineering, and an urban mass transportation expert who helped design New York's Interborough Rapid Transit subway system.

He also served as a lieutenant colonel during World War I in the Aviation Section of the Signal Corps.

[2][3] After graduating from Hillsdale College in Michigan and the University of Nebraska-Lincoln in 1897, Arnold set up his own company in Chicago at a time when railroads were converting their power sources from steam to electricity.

[6] Arnold also assisted in the conversion of electrified railways in Los Angeles and Pittsburgh, and the cable car systems in San Francisco.

[7] On December 14, 1917, Arnold transferred to the regular Army as a lieutenant colonel in the Aviation Section of the Signal Corps.