The remains were discovered by a quarry worker at Mepal on 16 May.
The man, aged between 30 and 50, could have died as long ago as the 1960s, but it is more likely his death occurred "about 10 years ago", police said.
It is thought the head came from a site 40 miles away in Sharnbrook, Bedfordshire.
Detectives have been working with specialists from the Natural History Museum and an odontologist (forensic dentist) to try to identify the remains since the discovery at the Block Fen Drove site.
The man's identity and cause of death are still not known. The National Crime Agency's DNA database has failed to come up with a match for the remains.
However, detectives are looking at the possibility he was hit by a train in Bedfordshire in "a tragic accident".
"We believe the head may have been lodged somewhere along the structure of the bridge in Sharnbrook, and as part of the recent cleaning process, has been dislodged, found its way into the skip and deposited at the quarry in Mepal," Det Insp Jerry Waite said.
The Mepal site where the head was found is used for landfill and recycling, as well as the supply of aggregates such as limestone and gravel, and mixing concrete.
A severed head found in a Cambridgeshire quarry last month has been identified as male, but his identity remains a mystery.