Plaque: Palace Gates Line
The Palace Gates Line ran through this site from 1878 until 1964 linking Seven Sisters Station with Palace Gates Station in Wood Green.
The plaque is on the pavement between the pairs of 'railway lines'.
Site: Palace Gates Line (1 memorial)
N15, Between Langham Place and Downhills Park Road
Londonist have a video about this line with this memorial saved to the end - there's a mural as well.