These are located in the area formed by the three stations of Earls Court, Gloucester Road and High Street Kensington which forms a rough triangle - hence the name.
They are easily viewable from a train as it travels between Earls Court and High Street Kensington.
Until the sidings were built 'over' (literally) in the late 1950's with the construction of the West London Air Terminal (the site is now occupied by a Sainsburys supermarket) they were in the open. In Piers Connor's book 'Going Green' there is an excellent view of the area, taken in 1935.
Today they are used to stable some of the C Stock trains used on the Wimbledon - Edgware Road service. Occasionally they are used by the Circle line, but this is only for the purpose of reversing a train back to Edgware Road. D Stock trains will not fit in any of the sidings; they are too long and would result in the points and signalling in the area becoming locked.
There are five sidings used by service trains 34 to 38 Roads. 34, 35 and 36 Roads are to the right of a train travelling towards High Street Kensington from Earls Court; 37 and 38 Roads are to the left. A train coming from High Street Kensington can only be routed into 34 to 36 Roads; a train reversing from the normally eastbound road between Earls Court and High Street Kensington can be routed into any of the sidings.
There is one further siding, but this can only be used by battery locomotives as there are no current rails. In any event this is too short even for a C Stock train.
Also in the area are a variety of facilities; an accommodation block used by the Train Maintainers who check the trains each night, accommodation for cleaners and their supplies and a Permanent Way staff room.
Photos of the area are pretty rare, and the following will show why I think! Very little daylight penetrates the area, but I hope these pictures will give a feel for what it is like.
In this picture we are looking towards High Street Kensington; the track to our left is the reception road which gives access to 34 - 36 Roads. The points giving access to the siding are visible in the centre of the picture.
Further into the depths of the area! Again looking towards High Street Kensington; an eastbound service is is visible heading in that direction.
As stated above, the sidings can only accommodate C Stock trains; but even then, the clearance between the stopping position and a very solid brick wall is but a few feet as this picture shows. Because of this there's a system installed whereby should a train overrun an alarm will sound in the station supervisor's office at Gloucester Road. This is 38 Road; the road number is just visible behind the graffiti that 'decorates' the wall......
Again looking towards High Street Kensington. The shunt signal visible is that for 37 Road (to the right) and 38 Road is to the left. The points which will allow a train towards High Street Kensington are also visible.
Taken from pretty well the same place, but with two trains passing; a D Stock to the left and a C Stock to the right.
Not the greatest collection of photos, I know - and even these have been enhanced to lighten them and make them more visible!