What we call the NEC today was electrified due to the requirements put on railroads doing business in downtown New York City, which wanted the railroads to switch to electricity to eliminate the smoke and pollution. 
It was also the smoke issue which lead to the Great Northern electrifying its tunnels through the Cascades, and the Boston and Main with the New Haven's help, to electrify the Hoosac Tunnel from 1910-1946. In both of these cases, it was diesels which lead to the demise of the electric operations since they were short-distance operations strictly for the tunnels, and the less pollution and operating costs overall by the diesels meant the electrics could be retired.