Four people have been injured and a fire has begun after a crash involving a freight train and truck near the Northern Territory town of Katherine.

Emergency services are responding to the incident on Florina Road in Cossack, about 10 kilometres south-west of Katherine, and have cordoned off a 5km area surrounding the incident.

Aurizon, the owner of the rail line between Adelaide and Darwin, confirmed in a statement that a truck had collided with one of their freight trains at a level crossing shortly before 11am on Wednesday.

"The four Aurizon traincrew have been transported to hospital and are in a stable condition," a company spokesperson said. 

"The status of the occupant/s of the vehicle is unknown at this stage."

The ABC understands the heavy vehicle involved was a side tipper truck, a type of dump truck mainly used in construction work.

According to NT Health, four men aged 32, 38, 47 and 49 were being treated at Katherine Hospital. 

Director of St John Ambulance NT, Andrew Thomas, said all four patients were in a stable condition, "with some minor head wounds and some limb injuries".

"One [other] person was seen at the scene and treated, but didn't want to go to hospital," he said.

Mr Thomas said the truck driver did not go to hospital.

Florina Road has been closed in both directions, but a detour via the service road has since been set up for most vehicles.

Authorities have warned that major traffic delays are expected.

Train carriages caught fire, causing 'significant' damage

NTFRS Deputy Chief Fire Officer Joshua Fischer told ABC Radio Darwin a fire had begun in the main carriages of the train before spreading to nearby bushland, but that the blaze had now been contained.

He said NTFRS, Bushfires NT, and Royal Australian Air Force personnel, along with a water bombing helicopter, had all been used to contain the fire.

"Crews are now assessing the incident scene, and determining the next steps as we work to fully contain the incident," he said.

Mr Fischer said authorities held concerns that some flammable materials on the freight train, such as diesel, could cause further fires.

The freight train was being used to deliver goods and freight from South Australia to Darwin, he said.

Mr Fischer said it would take some time to clean up the "significant amount of damage that has occurred".

"Multiple carriages have derailed from the … train tracks, as well as damage as a result of the incident to the train network," he said.

"So there's a significant amount of work to be done."

Mr Fischer said authorities were working to separate and remove the remaining carriages from the train tracks.

The Aurizon spokesperson said the railway between Tarcoola in South Australia and Darwin would be impacted during the "recovery process". 

"The incident will be the subject of an investigation by police and the Office of the National Rail Safety Regulator, they said in a statement. 

NT Road Transport Association executive officer Louise Bilato said the crash would "absolutely" interrupt the supply of freight in the territory.