(Adds details on Union Pacific assets, paragraphs 5-6, 8)
Aug 30 (Reuters) - Major U.S. railroads have warned it could be a long time before normal operations resume in the Houston area where Tropical Storm Harvey caused catastrophic flooding that overwhelmed roads, bridges and train tracks.
The closure of rail lines in the grain transport hub and nexus for cross-border traffic with Mexico presents a costly headache for customers ranging from automakers to farmers who use the lines to send ethanol, cereals and auto parts to and from Mexico or to be loaded onto ships.
The top two U.S. railroads, Union Pacific Corp and Berkshire Hathaway Inc's BNSF Railway, have suspended operations in the area affected by the storm, as has regional railroad Kansas City Southern.
"We don't have a historical precedent with this one," said Thomas Williamson, owner of Florida-based rail broker Transportation Consultants Co. "I expect service to be disrupted anywhere from two to six weeks."
Union Pacific, the No. 1 U.S. railroad, said that as of midday on Wednesday, sections of track were out of service in 18 of its 28 Houston-area subdivisions, and it was sending traffic along alternate routes through Longview and Dallas to avoid the Houston area.
The railroad said it was using helicopters and drones to inspect track and facilities in areas without road access with a goal to assess damage before week's end, and workers were reassembling major rail yards at Settegast, near downtown Houston, and Englewood.
According to a Midwest-based conductor at the railroad who spoke on condition of anonymity, parts of Union Pacific's Englewood Yard were flooded as of late Tuesday.
The company, which has been rerouting rail traffic away from Houston, said traffic entering or leaving through the north and east sides of Houston was still on hold, and its main line between Houston and San Antonio, to the west, was still out of service.
BNSF said in a Tuesday customer announcement that normal train flows in the area were unlikely to resume for "an extended period."
A spokesman at CSX Corp, the No. 3 U.S. railroad, which has so far been unaffected by Harvey, said on Wednesday it was "closely watching the weather conditions in Louisiana and the southwestern portion of our operating network." (Reporting by Nick Carey in Detroit and Eric M. Johnson in Seattle; Additional reporting by Jarrett Renshaw in New York and Michael Hirtzer and Karl Plume in Chicago; Editing by Bill Rigby and Peter Cooney)