(Adds Houston mayor comments, quotes)
By Amanda Orr
HOUSTON, Oct 24 (Reuters) - Heavy rains fueled by the remnants of Hurricane Patricia inundated many parts of Texas on Saturday, triggering flash floods, derailing a freight train and forcing evacuations in at least one county, officials said.
Cities in the state's flood-prone Gulf of Mexico region including Houston braced for potential floods as rain systems intensified by Patricia, now a tropical depression after crashing into Mexico's west coast as a powerful hurricane, pushed across the state.
Houston, the state's second most populous metropolitan area with 6.1 million people, was under a tornado watch until 10 p.m. (0300 GMT Sunday).
There were no confirmed deaths. In San Antonio, a woman reported that her boyfriend was swept into a drainage ditch by flood waters as he walked his dog early Saturday, and he was considered missing, officials said.
A flash flood in Navarro County, south of Dallas, was so forceful it swept a Union Pacific freight train off the tracks, pushing locomotives and some rail cars on their sides, according to a company spokesman and TV footage. There were no reported injuries.
Navarro County, about 50 miles (80 km) south of Dallas, was one of the hardest-hit areas. The town of Powell received 20 inches (50 cm) of rain in the past 30 hours, according to meteorologist Brett Rathbun of Accuweather, while county authorities requested sandbags for an unspecified number of homes being evacuated due to flooding, according to a statement.
Interstate 45 in Navarro County was shut in some spots due to rising waters, stranding some drivers.
Taylor Bates, 19, said she had not been home since Friday, when both of the roads to her residence in an unincorporated area of the county flooded and partially washed away.
"My mom is stuck at the house and she can't leave, but the house is OK," she said. "But I can't get home. Both ways to get to our house are flooded."
Some 75 water rescues from vehicles, homes and businesses had been carried out in Navarro County since Friday, Sheriff Elmer Tanner said.
"There have been a lot of evacuations but we don't have those numbers yet," he said. "We're just trying to keep everyone safe right now."
The weekend deluge marked the second time in five months that severe weather hammered parts of Texas. In May, a series of storms triggered floods and led to 21 deaths, prompting President Barack Obama to declare parts of the state as disaster areas.
Saturday's rainfall led to the cancellation of about 100 flights at Dallas/Fort Worth International Airport, one of the country's busiest air hubs, according to tracking service FlightAware.
In the Galveston area, authorities urged a voluntary evacuation of the elderly and residents with medical issues on the Bolivar Peninsula near Galveston Bay.
"Emergency responders may be unable to reach them," said Mark Henry, a judge and emergency management director in Galveston County.
The conditions could hinder transportation to and from the peninsula. Power outages are also possible as a result of gale force winds, authorities said.
National forecasters predicted 8 to 12 inches (20 to 30 cm) of rain will hit the coastal area by Monday morning, combined with tides up to 5 feet (1.5 meter) and wind gusts up to 35 mph overnight on Saturday.
Houston Mayor Annise Parker warned residents to stay away from wet roads after dark and be aware that flooding is likely.
"This is a situation that Houstonians should be familiar with," Parker said, referring to the city's vulnerability to flooding.
Even so, some residents eyed the coming storms with some indifference.
"In Houston it's always like that when it rains - it's going to flood," said Vittorio Preteroti, owner and chef of the upscale Perbacco Italian restaurant in downtown Houston, before the eatery opened for dinner.
Nonetheless, officials reminded residents of deadly flooding in the past and urged vigilance.
"Unfortunately some people lost their lives in high-water incidents," said Harris County Judge Ed Emmett, whose county includes Houston. "We're going to get a lot of rain tonight and it's going to result in some high water situations so for heaven's sake be careful out there."
Retailers in the area reported that locals made a run on supplies in anticipation of the floods.
"We've seen a spike in the sales of bottled water, bread, milk, canned meats, fruit and even items such as charcoal, which customers use to barbecue if there's a power outage at their homes," said Kristal Howard, public relations manager for Kroger Southwest. (Additional reporting by Alex Dobuzinskis in Los Angeles, Lisa Maria Garza in Dallas; Jim Forsyth in San Antonio, and Karen Brooks in Austin; Editing by Frank McGurty, Cynthia Osterman and Marguerita Choy)