WASHINGTON (AP) _ Following a string of train accidents in recent years, railroad tank cars carrying hazardous materials must be replaced, rebuilt stronger and travel at slower speeds over half of the nation's tracks under new federal proposals.

The Transportation Department's proposed rule requires tank cars carrying poison inhalation hazard, or PIH, commodities to be equipped with puncture-resistance protection to prevent penetration at speeds of 25 mph for side impacts and 30 mph for head-on collisions. Those speeds are more than double the limits for existing tank cars.

The proposed rule also sets a maximum speed limit of 50 mph for any train transporting a PIH tank car and a temporary speed restriction of 30 mph for cars not meeting the puncture-resistance standard that travel in non-signaled territory, or "dark" areas that comprise about half of the nation's tracks, Federal Railroad Administrator Joseph Boardman said during a conference call Monday morning.

Some of the oldest PIH tank cars also will be phased out on an accelerated schedule over concerns that they do "not adequately resist the development of fractures that can lead to a catastrophic failure," according to the government proposal.

A series of deadly railroad accidents involving hazardous materials including the 2002 derailment and chemical spill on the edge of Minot, N.D., a 2004 derailment near Macdona, Texas, and a January 2005 train collision and chemical spill in Graniteville, S.C. led the FRA to develop the new rules, which Boardman called a "revolutionary step forward in tank car safety."

The Federal Railroad Administration expects it will take the industry eight to years to replace the 15,300 tank cars, but half of each owner's fleet must comply with the enhanced standards within five years. Boardman said chemical shippers will bear most of the cost for the new $125,000 cars, a 47 percent increase over the current price tag.

The government estimates the cost to industry will be $350 million spread over 30 years, while savings from less property damage, casualties, litigation and other factors will total about $665 million. The agency did not forecast costs and potential savings during the eight years industry has to come into compliance, Boardman said.

A spokesman for the Association of American Railroads said the trade group would not comment until it had time to review the government proposal. Boardman said public hearings with the railroads will begin in mid-May.

While domestic railroad accidents and deaths fell last year, the number of incident reports involving hazardous materials jumped to 43 from 28 in 2006, according to government data released earlier this month.

Copyright 2008 The Associated Press.