Not much remains of the Western Horse Tunnels AKA the Camden Catacombs, cut off from the larger Eastern part, they still provide access to two locations worth checking out. One is the Grade II listed Stationary Engine vault, once used to house a pair of Steam Engines which would pull trains above up the steep incline and over the canal. The other is an mostly abandoned rail tunnel, never electrified and used to move empty steam train carriages, the 'Up Empty Carriage Line' also known as the Camden Rat Hole.
The following is quoted from a book "The Great British Railway Station, EUSTON" published by Irwell Pres in 1994. isbn 1-871608-28-7. Page 40. This burrowing line was the "rat 'ole" of evil repute and a crew would go to great lengths to ensure a clear trip through - stalling could be positively dangerous in its fume-laden close confines. It was the scene of at least one "rescue" in late BR days, when a train of empty stock from Willesden with a Fowler 2-6-2T stalled in "the 'ole". Fortunately one of the "twins", 10000 or 10001 (LMS diesels) was available and a loco inspector (who was later commended) took the diesel in, using the nose doors to gain access and couple up, drawing the train out into the living world.'
Below are some plans from the National Rail Archives with some minor edits from myself showing the site as a whole. The remainder of the Horse Tunnels are on the left, the large structure to the right is the listed Engine House and the rail tunnel can be seen running perpendicular on a lower level.
Looking down the shaft to the rail tunnel. The 25kv cables running down here and some plumbing are all the tunnel seems to be used for currently.
Looking West is the escape out towards Primrose Hill, you can see the faint outline of the tracks on Google Maps Satellite view if you look at the northern most lines near the Primrose Hill Tunnel entrance. Not sure of the function of the modern electronics box, there were a few electrical diagrams in there but not much else. East, the tunnel curves into darkness. I walked the tracks for a fair amount of time but didn't get to the other end, it opens out again and exits at what was once a depot near Euston. The sound of trains passing by on the surrounding lines would pierce the silence intermittently and were loud and clear enough that you'd forget the track was dead at times, the tunnels were pitch black silent excelt for a constant ringing noise, was pretty sinister down there by myself to say the least. :D
Below is a pic I took, like the lead picture, of the tunnels lit up with a number of lights spread down it's length.
Part way walking down the tracks I found a pile of huge encyclopaedias, can't imagine someone would have lugged them all down via the way I came so not sure how so many ended up here... One was left open on the page for death which gave me a good laugh... at this point with the weird ringing noise getting louder I decided to get out quickly. :D
Below you can see the Rat Hole Tunnel documented in the book 'Britain's New Railway' 1996. Located in the bottom right hand corner.
The second part of this explore I've only touched on so far due to the fact that it's mostly flooded, although I reckon might be only a few feet at this time, I need to measure it soon. Below-left is one of my photos showing the southern coal store for the engines and to the right is a plan from the National Rail Archive for the entire structure. 2 engines used to pull an endless rope which would carry trains up the steep incline to go over the nearby canal due to the inefficiency of train engines at this point in time. The main stair case I imagine is capped, can't see any evidence of it above ground, there is daylight coming from the southern boiler room so I think there may be an entrance on the side path of the tracks but it's not easy to confirm this.
Pictured below is what the site looked like above ground during the short life of the mechanism. The engines were built underground so as not to disturb the area's local affluent residents and tall chimneys built to siphon the fumes away. In less than a decade of function, technology had improved enough that steam engines could power their own way up the incline. The engines and associated machinery were sold off and the vaults were sealed up. You can see the exit from the engine vaults inbetween the two chimneys.
Below are two photos of the vaults Copyright of BWCP Architects, c1990. On the left is a rope tensioning room and on the right is the very central vault. Might be able to pursuade myself to check these out for myself but for now it's as far as I go.
Thanks for reading. :]