Matfield Green ? Cleanup efforts at a derailment site finished ahead of schedule, and Burlington Northern Santa Fe Railway said Saturday that trains already were able to travel on its premier line between Chicago and Los Angeles.

Investigators continued to look for the cause of Thursday’s derailment, which drove 250 residents from their homes because one of the cars leaked sulfur dioxide.

Along with the train line, Kansas 177 was reopened by Friday evening. It had been closed from Rock Creek Road to south of Matfield Green.

Spokesman Steve Forsberg said the railway had arranged a temporary fix in the area, allowing trains to travel the route at slower speeds. He said it is a standard procedure to place a temporary fix, then send engineers back for a permanent solution later.

The freight train that derailed during stormy weather was trying to stop at the time — just minutes after dispatchers told its crew that a tornado warning had been issued for Chase County, Forsberg said.

A severe wind gust likely hit the middle third of the 118-car train, derailing 39 cars, Forsberg said. It is standard practice to stop trains anytime a tornado warning or dangerously high winds are reported in the area, he said.

Investigators planned to examine the equipment and the track to determine whether there were mechanical or structural problems, Forsberg said.

The leak of the pressurized sulfur dioxide prompted the evacuation of 250 residents in a nearly 5-mile radius of the site. Most were allowed back to their homes after four hours. By Friday afternoon, the evacuation area was held to a one-mile radius of the derailment as crews dumped lime on the leaking rail car to neutralize the toxic fumes.

The Chase County Sheriff’s Office said Saturday that all residents had been allowed to return home.

The derailment was in a sparsely populated area in Chase County. The train was headed from Dayton, Texas, to Galesburg, Ill.

A Matfield Green man who complained of trouble breathing was treated and released from a hospital, while at least six firefighters and law enforcement officers were treated at the scene for respiratory ailments, sheriff’s dispatchers said.