The Post Office Department's ultimate goal was to provide coast-to-coast airmail service. By 1918 it had established service between New York and Chicago. After September 8, 1920, airmail was flown across country, from New York to San Francisco. In the early days of airmail service, the lack of ground lighting made night flying impossible. Mailbags were routinely removed from airplanes at night and placed on mail trains, which sped them on their way. The next morning the bags were put back on the nearest mail airplane to continue their journey. At it's fastest, transcontinental airmail service saved less than 2 days over mail sent the entire distance by train. To institute coast-to-coast airmail service, postal officials had to show Congress that round-the-clock flying was possible. If mail moved only slightly faster by air than by train, few in Congress would be persuaded to fund the service.