TUCSON, Feb. 15 -- A section of 47-year-old steel rail blamed for a Union Pacific freight train derailment near Sahuarita last month had passed three routine inspections in December, the Arizona Daily Star reports.

But inspectors probably could not have detected the "detail fracture" that led to a 10,000-gallon sulfuric acid spill on Jan. 2, a report by the Arizona Corporation Commission Railroad Safety Section concluded.

"It's an internal defect - you're not going to see that detail fracture," state rail safety supervisor Don Thompson said, adding that no state action against Union Pacific is planned.

Thompson said such fracturing frequently occurs due to expansion and contraction as temperatures rise and fall, noting that a track may stretch or shrink by 3 feet for every mile of track.

Based on "hammering" marks on the edge of the eventual crack, investigators believe that the straight rail section cracked through the top as an earlier train traveled over the section hours before the derailment, Thompson said.

The 19-car derailment prompted the evacuation of about 96 Sahuarita-area residents when derailed tanker cars spilled the sulfuric acid onto the ground. No one was injured in the incident, which caused $1.3 million in damage and cleanup costs.

According to the state report, the quarter-mile-long rail section that failed had been visually inspected by a Union Pacific track crew on Dec. 30, just three days before the derailment.

A few loose and missing bolts were found as the crew inspected about 50 miles of track on the Tucson-Nogales spur, but no problems were detected with the rail that failed, and the defects were corrected on the spot, according to the railroad's inspection report.

The railroad also inspected the track using special ultrasonic imaging equipment on Dec. 13, but no rail defects were found near the derailment site.

About a week earlier, the rail passed a routine visual inspection by a state inspector under by Federal Rail Administration rules.

Union Pacific spokesman Mike Furtney said the state report appears to confirm the company's own findings, but the company's safety officials were still studying the document.

"We do more than is required by the FRA in inspecting (track)," Furtney said. "In most cases, we will detect these defects, but not in every case as this case demonstrates."

According to the state report, the rail section that snapped was manufactured in Colorado in 1953, used elsewhere and laid at the site in 1986 as part of a practice of reusing old rails.

"It's allowable and it's pretty common," Thompson said. Furtney said old rails that are still functional are often moved to tracks carrying less traffic without problems.

"It's not unusual to have a 40- or 50-year-old track still doing the job effectively," he said.