A giant banner reading "Leave it in the ground" has been draped over the train bound for Drax near Selby.
One of the activists said the freight train was stopped using a red flag.
He said protesters had attached lines to the train's wheels to stop it moving. He said the intention was to start unloading coal using shovels.
A North Yorkshire Police spokeswoman said: "We've been aware that there are a number of protesters at Rawcliffe.
"British Transport Police and North Yorkshire Police are responding at the scene."