The southbound freight train, with 50 cars and six locomotives, struck the eastbound bus shortly after 7:30 a.m. at a crossing in Buffalo, said Gus Melonas, a spokesman for the Burlington Northern Santa Fe Railroad Co.
"The train blew its whistle, went into emergency stop, made contact with the bus, and we're further investigating at this point," Melonas said.
Fergus County Coroner Jerry Cloyd said two children on the bus were killed. Sheriff Eugene Couch said the two were brothers, but identities were withheld pending notification of relatives.
David Faulkner, chief executive of the Central Montana Medical Center in Lewistown, said three children and the driver were brought to his hospital.
The driver was reported in serious condition in the intensive care unit. One child also was in serious condition. Two children were treated and released.
No names were released, but Faulkner said the children were about 6 or 7 years old.
No one on the train was injured.
The Montana Highway Patrol said the two dead and four injured accounted for everybody on the bus, which belonged to the Hobson school district and was running a rural route in Montana's ranch country.
Couch said there was no obvious explanation for how the collision occurred. He said that while the area got some snow Monday night, the weather was clear at the time of the accident.
He said an investigation team from the National Transportation Safety Board was en route to the scene.
Buffalo is located in central Montana about 100 miles southeast of Great Falls.
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