Authorities are trying to establish what caused a fire that destroyed a historic railway structure and blocked rail services between Sydney and Brisbane.

The blaze in the 76-year-old coal loader lit up the skies of Casino on Sunday night.

It came after fire crews had been called to a grass fire in the vicinity of the Casino Railway yard on Sunday afternoon.

They said they had left the blaze under control.

"[The fire] was in the [coal loader] when we arrived so I don't know whether it was in the structure [first] and caused the grassfire or vice versa," Fire and Rescue NSW Casino officer Greg Miller said.

Crews were called back in the early hours of Monday.

"When we got here [the hopper] was too hot to approach, it's quite hard to access and we could not get close enough," Captain Miller said.

"We made the decision to stand back and let the fire come down because it was not safe for crews.

"It's very hot inside and it will probably take days to fully extinguish."

The buildings at the Casino railyard are heritage-listed and the coal loader, built in 1947, was the last structure of its type in NSW.

"It was a beautiful old structure," Captain Miller said.

The fire has closed the rail line at Casino and Sydney to Brisbane rail freight and passenger services are on hold until further notice.

An Australian Rail Track Corporation spokesperson said the rail line had been cleared of debris but a structural engineer would need to attend the site to inspect the coaling tower before train services could resume.

Fire warning

Sunday afternoon's grassfire in Casino was one of almost 200 grass and bushfires to burn throughout NSW at the weekend.

The Rural Fire Service (RFS) said it was becoming increasingly concerned about the number of fires that began as private hazard reduction burns.

It said more than 600 fires had been caused by escaped burns since the beginning of August.

RFS operational officer Angus McDowall said crews in the NSW Northern Tablelands were observing an "alarming number" of unsupervised burns, particularly around Glen Innes and Tenterfield.

"We have issued a number of formal cautions and warnings for a number of fires that have escaped," he said.

Meanwhile, crews have responded to more than 200 fires in the Kempsey and Nambucca Valley local government areas in August.

"Quite a substantial amount of those have been from land owner burning," RFS Lower Mid North Coast officer Liz Ferris said.

Superintendent Ferris said three consecutive La Nina weather events had created a high fuel load which posed a heightening risk amid the increasingly dry conditions.

"We want to encourage [land owners] to proactively reduce those fuel loads, but also to be able to do that safely," she said.

Landholders who fail to notify fire agencies and neighbours of burns can face fines of up to $5,500 and/or 12 months in jail, while escaped fires have penalties of up to $132,000 and/or seven years in prison.