TRAIN safety is receiving fresh scrutiny in the wake of three major rail accidents in the United States.The deaths of at least seven people in an Amtrak derailment Wednesday in South Carolina are focusing attention on the need for improvements along passenger lines. For freight haulers, tighter control over the transport of hazardous materials is likely after two recent accidents in California that resulted in significant chemical spills. Although different circumstances were involved in the three incidents, experts say they highlight gaps in regulations and industry safety programs. "I'm sure whatever the weaknesses are, there will be improvements," says William Pugh, former head of the rail division of the National Transportation Safety Board. The accidents come at a time when there have been positive trends in train safety. In the 1980s, the number of train wrecks in the US decreased 64 percent, according to the Federal Railroad Administration (FRA). Train derailments, the most common type of mishap, dropped by 67 percent. But Amtrak, the nation's main passenger carrier, has seen its accident rate edge up in recent years - from 57 in 1986 to 113 last year, according to FRA figures. Before Wednesday's wreck, the carrier had not recorded a fatality along its lines in more than four years. The crash, in which a section of a Miami-to-New York train jumped the tracks, was Amtrak's deadliest derailment since 1987. The industry's record on transporting hazardous materials has been mixed. The volume of toxics hauled by rail has in creased by 50 percent in the past five years. Although there have been fewer mishaps involving trains carrying toxic materials in recent years, incidents in which damage to rail cars resulted in release of chemicals into the environment have increased. Transportation Secretary Samuel Skinner said this week that "there is no indication of a systematic problem in railroad transportation." But environmentalists and some lawmakers see warning signs - particularly when it comes to hauling hazardous substances. "How many warnings do we need before we clean up our act?" queried US Rep. Barbara Boxer (D) of California in congressional hearings this week. A derailment and chemical spill that befouled dozens of miles of the Sacramento River in northern California last month focused attention on the classification of chemicals. Materials registered as hazardous by the US Department of Transportation (DOT) are subject to strict labeling, packaging, and transportation rules when being moved by rail or other means. But hundreds of agricultural chemicals - including the pesticide metam-sodium that killed most of the aquatic plants and thousands of fish in the Sacramento River - are not included on DOT's list. Environmentalists and some lawmakers are pressing the federal government to expand the list. "There are whole categories of toxics that have been exluded," says John Cameron of Citizen Action, an environmental group. Even when chemicals are regulated, critics say, problems can arise. The train that broke an axle and derailed in Ventura County north of here Sunday, blocking traffic on a major north-south highway, leaked hydrazine, a corrosive chemical. Although the substance was labeled hazardous and carried legally in 55-gallon drums, critics believe sturdier containers should be used. Emergency crews that responded to the spill complain there wasn't enough information in documents on the train about the chemicals that leaked out - charges South Pacific Transportation Company, owners of both trains that derailed in California, have denied. State officials are pressing for greater disclosure of the hazardous substances moved by rail in California. In the past, some groups have suggested a computerized system be established to catalog the chemicals. But railroad officials say it would be too costly and difficult to set up. "It has never been made clear how a computerized network would improve safety," says Jim Reiter of the Association of American Railroads.