Great Western Coffee Shop

There has always been a smell just west of Southall. This usually smelt like a gas leak from the gas-holders. But now they seem to have large water sprays all along the track that smells like raw 'sh$t.' It even permeates the aircon units on the trains. WTF is is going on?

Do you mean beside the gasworks site? The usual reason for water sprays is to lay dust during demolition or  construction, but the obvious process that would call for it - washing the top few metres of the site - happened a year ago. There were complaints of the smell then, though labelling it as tar-like. The fact that Southall has an asphalt factory (though the wind is not coming from that direction just now) was also part of that story (http://southall gasworks site). But if construction itself has now started, which I've noticed these days often means moving big heaps of soil back and forth across the site for no obvious reason, that might be the reason for the sprays. Mind you, nice washed-clean soil ought not to smell.

They use water sprays for damping down asbestos dust which always seemed to me to be ineffective if the water sprays along the line west of Southall Station are anything to go by the smell (and dust?) well permeates the passing trains.

That site used to be part of a massive Coal Gas works site whereby coal from S.Wales was bought down and distilled in huge retorts. My Dad remembers it working in the 70's before the likes of North Sea gas rendered it uneconomic.
I expect that 100 years worth of tar, creosote and naptha, that has been disturbed,  is the stink that you can smell! :o

In May they started dismantling the massive blue gasholder (https://flic.kr/p/26nA4AG) which has been there since the 1930s :'( :'( :'( (state of progress last week (https://flic.kr/p/27pjNsZ). Some of the smell may be from there, but the asphalt factory built a few years ago near the A312 bridge is also responsible for a lot of complaints in the area.

But how will lost aircraft now find Heathrow?  :o

The 'LH' and arrow painted underneath, near the top of the gasometer in the above photo points towards Heathrow in an attempt to stop them landing at the nearby RAF Northolt airfield by mistake.  It has happened in the past! (https://abpic.co.uk/pictures/view/1001607/)

The point being that there was another, similar-looking, gasholder at Roxeth (South Harrow) at a similar bearing and distance from Northolt. After that embarrassing (for the pilot) incident that one had "NO" painted on it to match the "LH". The Roxeth one was taken down many years ago - the site is now a Waitrose.