You'll not spot in on the tube map, because the tube map doesn't do shapes. But there's a near-equilateral triangle of railway lines near Gunnersbury, whose sides comprise two branches of the District line and a bit of the Overground. It's quite a big triangle, encompassing six acres all told, packed full of one of London's more unusual nature reserves. You can see it from the train, if you know what you're looking out for and where. But you don't enter from the Gunnersbury end, because that would be too obvious.
The way into the Gunnersbury Triangle nature reserve is very close to Chiswick Park station. A short distance along Bollo Road (named after a lost river, no less) is a gap in the wall and you just wander in. This is the volunteer hub of the operation, with a hut to boil a kettle, and presumably somewhere to entertain school groups. A group of lovely ladies were busy maintaining stuff when I arrived, offering a smile rather than an officious welcome. Head down the slope, and round the corner and you're in. This is the eastern tip of the triangle, and the one point where the main path runs right up alongside the railway. This is the District line round to Richmond, as the tracks descend from the viaduct after Turnham Green, where a big sign announces the presence of the reserve to all who care to look. It's easier to read in winter, I'd expect, and the fox on the logo looks a little pinker now than the brown I suspect he started out.
If you've picked up a leaflet, or if you can follow the posts in the ground, a Triangle Trail leads you round the site. Don't worry otherwise, most of the points of interest are labelled, from the silver birches in the introductory woodland to the insect-friendly towerblock habitat made from five stacked pallets. Things get a little willowier by the frog pond, which is pretty but nothing extensive, and susceptible to drying out during periods of summer drought. Lift a stump to spot a stag beetle, signs urge, which is just one of the child-friendly activities if you have a toddler who likes urban jungle.
In the southwesternmost part of the triangle is a clearing created by coppicing, on the face of it less interesting than the preceding woodland, but a different kind of space more suited to wild flowers, caterpillars and other creatures. Straight ahead is the farthest corner of the site, not immediately accessible by path but not entirely overgrown by undergrowth either. This is the point where District meets Overground, and it feels a little odd to be on the inside of the tracks for once. There's a much clearer view of the yellow trains from a mound further north, once an abutment on which a bridge into the site rested, now accessed via a recently repaired wooden staircase. Close by is the reserve's largest pond, created on the site of a former gravel working, and currently the prettiest location on site. Purple loosestrife blooms above the boardwalk, and a variety of insects skim across the waterline.
In the northwest corner is an extensive area of meadow, mostly fenced off to prevent incursion by humans, which I scanned in vain for green woodpeckers and voles. The path then bends round to follow the third side of the triangle, a dusty track which has an unexpected history. This used to be a railway line, a link on the London & South Western allowing passage from South Acton to Hammersmith. But the ActonCurve closed in 1965, long enough ago to have almost completely vanished, but just late enough for it to form the precise boundary between the boroughs of Ealing and Hounslow. The acidic soil provides yet another different habitat for nature to colonise, from butterflies to oak saplings, as further signs along the path make clear.
Immediately to the north, on what used to be industrial land, a mixed use development has recently encroached. These flats could look a lot worse, but it's still a bit of a jolt to see them up close to what's otherwise a green and rural sanctuary. Equally it's rarely quiet here, as trains rattle by on all sides with consummate regularity. You'll not see most of them, the woodland's thick enough, plus the one remaining railway line to the north is some distance away on the other side of Bollo Lane. Whatever, this is a site to treasure, and hopefully Gunnersbury's green triangle will hold further encroachment at bay for many years to come.
Meanwhile I can't leave the area without waxing a bit lyrical about Chiswick Park station. This opened in 1879, but was substantially rebuilt in the early 1930s when the Piccadilly line pushed through. The District line platforms were set back to allow the passage of two non-stop tracks down the middle, and it's a long way across from one to the other. Passengers wait beneath cantilevered concrete canopies, possibly finding space for a sit down inside a narrow glass shelter. It's a bit bleak, or else it's entirely charming, depending on your point of view. But surely everybody loves the main station building, one of Charles Holden's finest, with its double-height brick drum somewhat reminiscent of Arnos Grove. The curvature is offset by a stout tower topped off by a roundel, rising high enough to announce the existence of the station to those on Chiswick High Road. I'm convinced it's impossible to take a decent sunlit photo of the station from the other side of the mini roundabout without getting at least one car, parked-up bike or loitering pedestrian in shot, but maybe I just haven't hung around for long enough yet. I haven't hung around for too long inside the ticket hall either, because there's only so long you can pretend to be interested in the windows at the hairdressers or the dry cleaners before station staff get suspicious. But the space is gorgeous, an unexpected horseshoe with a gold-lit roof. One arm being a complete dead end is the only clue that Holden didn't built it this way, the central ticket office is a slightly more modern addition, but thankfully a sympathetic one. A little further on are a pair of cream-enamelled heritage signs dating back to the 1930s announcing the major stations in each direction. The Westbound remains unsullied but a plaque below the Eastbound warns passengers that this sign has been retained for heritage reasons and "Mark Lane station is now called Tower Hill". Not that this has stopped some employee at some point in the station's history from slapping a "Tower Hill" nameplate over the top of "Mark Lane", in not quite the right sized font and slightly indented, to destroy the illusion. But the rest of Holden's vision at Chiswick Park stands as testament to his architectural imagination. And at least it isn't Gunnersbury.