From the Wikipedia page [1]
Highgate Road railway stations were two stations of the Tottenham & Hampstead Junction Railway lying virtually side by side in the north of Kentish Town in North London on separate lines which joined east of the stations.
High level station
The High Level station was on what is now the Gospel Oak to Barking Line. It opened in 1868 and closed in 1915. At one time a link led to Kentish Town Station on the up Midland Main Line.
Low level station
The Low Level station opened in 1900 and closed in 1918. It was on a line which left the Gospel Oak to Barking Line somewhat east of the High Level station at Junction Road Junction from which road the slightly later Junction Road Station took its name. Links from it led to the up Midland Main Line and to the North London Line heading east. Part of the route of the NLL link was later lowered for a connection to be made to the down MLL.
See also Highgate Station