Years back, when travelling from Willesden Junction through to Camden Road via Primrose Hill (I think it was when the NLL wasn't working), I noticed a rusty single line on the north side of the line just after leaving the tunnel from South Hampstead, which went down into a tunnel. I later discovered that this was the Up Empty Stock Tunnel which ended up on the western side of the lines into Euston. I presume that it must have gone under the Regents Canal as well as under all the tracks on Camden Bank. The track has since been removed. Does anyone here know anything of the history of this line, when it came into use and when it was abandoned?