DALLAS - A powerful storm system rumbled through Texas on Saturday, flooding roads and causing a freight train to derail as parts of the state braced for the remnants of Hurricane Patricia to arrive.

Many parts of Texas, including its biggest cities, were under flash flood watches through Sunday or Monday. The remnants of Patricia, which was downgraded to tropical depression status and was expected to reach northern Mexico by Saturday night, may add to the rain falling in South and Central Texas, said Jesse Moore, a National Weather Service forecaster in Fort Worth.

Southeast Texas, including Houston, was under a flash flood warning until early Sunday morning, according to CBS affiliate KHOU. Meteorologist David Paul said the heaviest rain would last until about 3 a.m. with lighter showers after that.

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"There will be localized flooding in Houston, primarily street flooding," Houston Mayor Annise Parker said Saturday afternoon, adding people should be "prepared to be patient."

"Spend some time wherever you are if that area is experiencing particularly heavy rainfall," she said.

Nearly 3,300 customers were without power late Saturday in the Houston area, KHOU reported.

Galveston County Judge Mark Henry on Saturday issued a voluntary evacuation for Bolivar Peninsula, just northeast of Galveston Island, after forecasters predicted that the area would get 8 to 12 inches of rain and tides that are 4 to 5 feet high.

The judge warned that residents who don't leave might find themselves cut off from emergency services as the heaviest winds and rains come ashore Saturday evening. But county spokeswoman Brittany Rainville said they don't think very many people chose to evacuate. She said the county had two buses waiting all day to evacuate people but no one showed up.

The storm system already moving through Texas dumped more than a foot of rain on parts of the state on Friday, causing flooding that blocked several major roadways.

In San Antonio, a man walking his dog before dawn early Saturday was swept into a flooded drainage ditch and disappeared, fire officials said. Firefighters searched for two hours but had to stop due to bad weather. They planned to resume on Sunday, searching Woodlawn Lake for his body. The dog is safe.

A driver in the Central Texas town of Temple heading to work Saturday morning was saved after he was able to get out of his car that was floating in floodwaters and grabbed a tree. Temple police say the man called the emergency dispatcher from his cellphone to summon help. Firefighters retrieved him and walked him to safety.

A Union Pacific freight train derailed before dawn on Saturday near Corsicana, about 50 miles south of Dallas, because a creek overflowed and washed away the tracks, said Jeff DeGraff, a railroad spokesman. The two crew members swam to safety and nobody was hurt, he said.

A Union Pacific train derailed near Corsicana, Texas, after floodwaters washed out the railroad track, Saturday, Oct. 24, 2015. KTVT

"They (crew) escaped the train after it stopped and swam to high ground," DeGraff said. "A Navarro County rescue team was able to get in and pull them to safety, they are back safe on dry ground."

One locomotive and several rail cars loaded with gravel went into the water and were partly submerged, DeGraff said. He had no specifics on how many cars derailed because crews couldn't reach the site due to flooding.

Authorities on Saturday morning reopened a section of Interstate 45 near Corsicana that was closed overnight due to flooding, backing up traffic for 12 miles. Moore said that since Friday morning, Corsicana has received 18 inches of rain and Powell has received 20 inches.

"We have numerous, numerous roads within the county that are either under water, have been washed out," Navarro County Sheriff Elmer Tanner said.

Emergency rescue worker Daniel Cobb told CBS News he has dealt with all kinds of calls over the last 48 hours.

"The majority of them were vehicles, people trying to drive through, you know, standing water or rushing water," Cobb said. "It was just too deep and stalled the vehicle out, then they're in a situation they can't get out."

A woman holds a young girl as they stand near a road flooded by Bull Creek in Austin, Texas October 24, 2015. REUTERS

Flight tracker flightaware.com reported that about 100 Saturday flights had been canceled at Dallas-Fort Worth International Airport.

A flash flood watch was in effect for the Dallas, Fort Worth, Waco, Austin and San Antonio areas through Sunday morning, according to the National Weather Service. Corpus Christi, Laredo and Brownsville are under a flash flood watch through Sunday night.

Forecasters say Houston and Galveston remain under a flash flood watch through Monday morning, with possible coastal flooding concerns.

Patricia was the most powerful hurricane on record in the Western Hemisphere and made landfall Friday along Mexico's Pacific Coast as a Category 5 storm. It quickly lost power as it moved inland and appeared to have caused remarkably little damage.