This has been a public bridleway for at least 35 years since I lived in Rushton as a kid. The no "unauthorised" entry signs at the south end were put up a few years ago by the security company looking after the electric train track equipment but it is still a bridleway leading all the way to Manor Road in Rushton.
The council definitive map can be viewed here: https://www.northnorthants.gov.uk/rights-way-and-searches/definitive-map