Greenpeace activists have brought a coal train to a halt near Retford, using a model of a polar bear to block the track.
The organisation has commandeered a coal train and is urging the UN Climate Summit to "end the age of coal".
British Transport Police are on the scene to deal with a reported 50 protesters by the crossing at
A statement on Greenpeace's website says: "By doing this we're sending a message to the political leaders taking part in the UN climate summit today: end the age of coal.
"Stopping a coal train may seem drastic. It's not a decision I've taken lightly. But I strongly believe that if we are going to stop rampant climate change, drastic actions are needed.
"Where these actions are needed most is in the cabinets and boardrooms of world leaders. Right now, they're meeting in New York to talk - including David Cameron.
"Coal is one of the dirtiest fossil fuels available and it's not only terrible for the climate, it's also a major air polluter. The UK's ancient coal stations cause an estimated 1,600 premature deaths every year and cost the taxpayer up to £3 billion in health impacts."
The organisation is calling for the UK to scrap its interest in the coal industry in favour of "clean, green, renewable energy".
A spokesman for British Transport Police said: "We were called to a crossing at
"Officers are currently responding to the incident together with colleagues from Nottinghamshire Police."
A Network Rail spokesman said: "We have had to hold some freight trains back off the line. We have a normal process in place to deal with that. Or control centre is managing freight services around the area and some are being held back.
"This is a freight-only line so will not affect any passenger trains and we understand that the British Transport Police and Nottinghamshire Police are on the scene."
