Emergency services were called to the Bodmin and Wenford Steam Railway after the accident at about 1130 BST on Friday.
Police said the man, from Weston-super-Mare, was crushed by a steam crane. He died at the scene.
The Railway Inspectorate has been advised and its officers are attending the scene.
Annual check
The site is a well-known local tourist attraction, operating steam trains on short trips.
Most of the staff are volunteers and enthusiasts and all repairs and servicing of the locomotives and rolling stock are conducted on the premises.
The railway operates a steam-powered rail crane, which is used for track laying and other lifting.
It is annually inspected by an accredited Royal and Sun Alliance inspector on behalf of the railway's insurers.
During the course of its annual inspection, the inspector was in a position on the crane between a jib and a cable drum winch while the crane was being operated.
He became trapped and crushed to death between the two items of apparatus.
Police said the casualty was a middle-aged man from Weston-super-Mare.
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