A major spill in South Salt Lake earlier this year and other recent accidents highlight the risk posed by the chemical shipments, particularly in an age of heightened awareness of terrorist attacks - and have prompted mayors and some senators to demand action.

But an analysis of federal records by The Salt Lake Tribune shows the safety record for hazardous materials shipped by rail has improved in recent years. In Utah, the number of spills and mishaps has fallen steadily over the past decade, from 55 in 1995 to just nine last year.

There were three incidents in the first half of 2005 - including one the most serious hazardous material spills in the state in at least 12 years.

On March 6, emergency crews were called to South Salt Lake to respond to a tank car belching black smoke. The car had sprung several leaks, dumping caustic chemicals.

The tanker was supposed to be carrying sulfuric acid. But officials determined it had at least six other chemicals, including hydrofluoric acid, which may have eaten through the inside of the tanker.

More than 6,600 people were evacuated while emergency crews wrestled with how to handle the chemical brew. Federal officials are in the process of deciding what, if any, charges to file over the spill.

That incident was the exception. Just three spills in Utah since 1993 have required residents to evacuate.

Salt Lake City Mayor Rocky Anderson said chemical-laden trains rolling past homes and neighborhoods are no more dangerous now than they have been for years, but there is still cause for concern.

He is particularly troubled by the traffic along the 900 South line, which had been abandoned for years until Union Pacific reactivated it in 2001.

In December 2002, seven cars derailed on the line, spilling small amounts of lime. Other cars, carrying sodium cyanide and anhydrous ammonia, remained intact. Had they exposed their toxic, flammable cargo, it would have forced an evacuation of residents in a one-mile radius.

The city is working on eliminating traffic on the line by straightening out a tight curve that forces trains to slow down as they enter the city, leaving rail cars, some with toxic cargo, stuck in a bottleneck. Federal funds have been promised for the project and once it is complete, Anderson said, Union Pacific has agreed to stop traffic along 900 South.

Nationally, the number of rail incidents involving hazardous materials fell from 1,155 in 1995 to 753 last year, which includes anything from derailments to leaking containers found during inspections.

"Generally speaking, if you're looking at the safest way to ship hazardous materials, it would be by rail," said Tom White, spokesman for the Association of American Railroads, an industry group. "You do, unfortunately, still have accidents. You try like hell to avoid them, but occasionally they still happen."

White said there are several reasons for the reduction in incidents, including better-designed rail cars and better training for rail workers.

Despite the improved safety record, there is concern in Congress, among mayors and some other experts that the preparation to prevent a terrorist attack has been inadequate.

In 2002, warnings were reportedly issued to law enforcement agencies that terrorists may be targeting rail lines, based on information gleaned from interrogations and photographs of rail crossings and train cars found in an al-Qaida raid.

Richard Falkenrath, who was deputy homeland security adviser to President Bush until last year, said shipments of dangerous chemicals pose a uniquely deadly and vulnerable terrorist target and casualties from an attack on toxic chemical shipments could dwarf those from the Sept. 11, 2001, terrorist attacks.

"Thousands is conservative," said Falkenrath, now with the Brookings Institution.

Given the dire consequences, protecting the shipments should be of paramount importance, but the government has failed to take the aggressive action that is needed, Falkenrath said.

Carrie Harmon, regional spokeswoman for the Transportation Security Administration, said TSA, Homeland Security and others are working to increase safety, adding funds for tank car inspectors, providing additional money for grant programs and expanding a program that uses bomb-sniffing dogs at transit hubs.

"We're all constantly assessing and evaluating threats or potential threats to the transportation system in general and making an attempt to allocate the resources we have in the best and most efficient way possible," Harmon said.

The industry has done its own studies of security risks after Sept. 11, and made a series of security improvements, White said.

"Are there additional things that could be done? I suppose there always are. But we have taken an awful lot of measures to improve safety and security," he said.

Some are demanding more action.

The City Council for Washington, D.C., where trains run just a few blocks from the U.S. Capitol, passed an ordinance in February banning hazardous material shipments inside city limits, though the appeals court has blocked it pending a challenge by rail carrier CSX Corp.

The industry and Bush administration have sided with CSX in the case, arguing that rerouting shipments could force trains to travel greater distances. White said that would increase their exposure for attack or an accident.

The U.S. Conference of Mayors has asked Homeland Security Secretary Michael Chertoff to require mayors to be notified when hazardous materials are moving through their towns.

They cited the January derailment of a train carrying chlorine in Graniteville, S.C. that killed nine, injured hundreds and forced the evacuation of thousands. The mayors compared it to the effects of a weapon of mass destruction.

"These types of trains run on tracks through the hearts of our cities," wrote Akron, Ohio, Mayor Donald Plusquellic, president of the conference. "Our citizens should have a reasonable expectation that hazardous materials are being shipped in the safest manner possible and that local first responders are aware of such shipments in advance."

White said granting advance notice would inundate mayors with so much information it would be meaningless.

In past incidents in Salt Lake, Anderson said, emergency crews have had access to shipping manifests, although he expressed concern at the recent spill where the manifest was incorrect.

The stakes may become even higher for Utah. In addition to the thousands of shipments of hazardous chemicals, the state faces the prospect of between 10,000 and 20,000 casks of high-level nuclear waste rolling through the state on rail cars bound either for Yucca Mountain, Nev., or the Skull Valley Goshute Indian reservation, where a private storage site is proposed.

Anderson says the number of shipments makes an accident almost inevitable.

With that risk, the waste should stay where it is, an option supported by Gov. Jon Huntsman Jr.

Sens. Joe Biden, D-Del., and Jon Corzine, D-N.J., have each introduced legislation that would demand additional measures from the Department of Homeland Security.

Biden's bill would require re-routing the most dangerous shipments around "high-threat corridors;" developing a notification system for local officials; encouraging research to improve chemical tankers; and providing funds to train emergency personnel.

"The current state of our rail security system is worse than an accident waiting to happen, it is an open invitation to terrorists," Biden said.