Multiple red hand prints found in a cave near Glenbrook train station, that an expert said were Indigenous and "culturally significant", have been exposed as fakes created by teenage brothers in the 1960s.

The rock art was discovered during work to remove a 20-tonne boulder that threatened the Blue Mountains railway line, west of Sydney, in late March. It extended the weeks of delays on the train line as buses replaced services.

Specialist crews work to secure the boulder above the rail line at Glenbrook. Credit:Brenda Cunningham-Lewis/Blue Mountains Gazette

An archaeologist, with a specialty in Indigenous heritage, said at the time that the hand prints were "culturally significant".

But in a letter obtained by the Blue Mountains Gazette last week, the non-Indigenous man who created the artwork 50 years ago said he and his brothers were paying homage to local Indigenous culture and he was now keen to "set the record straight".