Because of its (relatively) economically isolated location, Alberta relies heavily on transportation links with the rest of the world. Alberta's historical development has been largely influenced by the development of new transportation infrastructure, (see trends below). Alberta is now served by two major transcontinental railways (CN and CP), by three major highway connections to the Pacific (the Trans-Canada via Kicking Horse Pass, the Yellowhead via Yellowhead Pass and the Crowsnest via Crowsnest Pass), and one to the United States (Interstate 15), as well as two international airports (Calgary and Edmonton). Also, Alberta is connected to the TransCanada pipeline system (natural gas) to Eastern Canada, the Northern Border Pipeline (gas), Alliance Pipeline (gas) and Enbridge Pipeline System (oil) to the Eastern United States, the Gas Transmission Northwest and Northwest Pipeline (gas) to the Western United States, and the McNeill HVDC Back-to-back station (electric power) to Saskatchewan.