AKRON, Ohio (AP) _ About 40 people driven from their homes by a freight train wreck were allowed back early today, ending the second evacuation in two days.

Investigators said speed, rough track and a faulty car may have been factors in the 21-car derailment Sunday in which two tankers containing 25,000 gallons of highly flammable butane caught fire and forced the evacuation of 2,000 people.

Hours after residents were allowed back in their homes Tuesday, a tanker jumped the tracks again as it was being moved, causing no fire but prompting another evacuation that ended after the car was moved to a rail yard.

Seven tankers are to be loaded onto flatbed cars later this week.

''It's wonderful to be back,'' said Jennifer Graner, who returned to work today at a business near the second derailment site.

National Transportation Safety Board investigators said the city of 230,000 narrowly averted disaster in the CSX Transportation Corp. derailment and explosion next to a B.F. Goodrich Co. chemical plant.

Chemicals stored outside the plant in large tanks and piped into the building include highly flammable butadiene and styrene as well as acrylonitrile, which can produce toxic fumes, said NTSB member Jim Burnett Jr.

''That's one of the reasons for the high level of safety board response to this situation, because the potential for a catastrophic accident did exist here,'' he said.

The safety board found ''some rough track conditions and alignment difficulties'' on the tracks, which are owned and maintained by Conrail and were last inspected by the railroad on Feb. 23, Burnett said.

The tracks next to the Goodrich plant had a speed limit of 40 mph, while tapes from the train place its speed at 43 or 44 mph, Burnett said. He said the train crew reported it was going 39 mph when the accident occurred.

CSX spokesman Lloyd Lewis, asked if the train may have been going at an unsafe speed, said, ''If the speed limit is 40 miles per hour, which it was, and the speed tape says 43 or 44, you can conclude that. It's like a highway.''

Burnett said the engineer, conductor and two brakemen were tested for alcohol or drug use. The tests were sent to a lab for analysis.

He said investigators are also studying whether a hopper car that carried sand may have played a role. It had 2 1/2 inches of excess lateral movement, which Burnett said ''is more than good maintenance practice would deem ideal.''

He added, ''We understand that this car was subject to some major maintenance in October of last year and this was only the second trip made with that car since then.''