Railroad electrification in the United States began at the turn of the 20th century and comprised many different systems in many different geographical areas, few of which were connected. Despite this situation, these systems shared a small number of common reasons for electrification. Most of the systems discussed in this article are either no longer electrified, or are now part of the Northeast Corridor and Keystone Corridor systems used by Amtrak and several commuter rail lines. A few isolated systems operate exclusively for hauling coal from mines to power plants. Most mass transit, streetcar, and interurban systems electrified very early (many from the beginning) but are not within the scope of this article. The Association of American Railroads opposes electrification due to its high capital costs. Impetus for electrification Laws banning steam locomotives (smoke abatement) A number of municipalities passed laws in the early part of the 20th century forbidding steam locomotives from operating within city limits, after some bad accidents caused by the awful conditions of visibility in smoke and steam-filled tunnels and cuttings. The most prominent of these laws was for New York City in 1903 (effective 1908). An extensive study was also undertaken in Chicago of the problems of smoke and the feasibility of electrification as a solution. Long tunnels Long, deep tunnels provide poor ventilation for steam locomotives, to the point where crews had to wear oxygen masks to avoid asphyxiation. The ventilation problem also limited the frequency of trains through these tunnels. The Cascade Tunnel is a good example. Also see the proposed North–South Rail Link. Mountains The electric locomotive has many advantages in mountainous terrain, including better adhesion, greater power at low speeds, no requirements for fueling or watering, and regenerative braking. The planned California High-Speed Rail system, for example requires electrification to achieve acceptable speeds through the Tehachapi Mountains. Traffic density Extremely high-traffic lines can readily recoup the high capital investment of electrification by the savings accrued during operation. The savings typically result from improved utilization of trains, and lower maintenance costs. Short-distance commuter operations Suburban commuter trains are an ideal subject for electrification since electric multiple units possess rapid acceleration, fast braking (sometimes regenerative braking) and the ability to change direction without running a locomotive around. It also reduces diesel locomotive emissions in relatively high-density areas. Freight operations Heavy freight trains are ideally suited to electric traction due to the greater pulling