I travelled on SWR from Whitchurch to Waterloo and back last Friday.
10.39 from Whitchurch: just three coaches, and crowded. With only one train per hour on the West of England line instead of the normal two, there must have been spare units which could have been used.
16.25 from Waterloo: three coaches. Crowded. At 16.20 the guard announced that the train was cancelled. There must have been well over 300 passengers who all got off. I asked the guard why it was cancelled. "Because of the heat," she replied. I pointed out that many other trains were running. "But they are electric, and this one is a diesel," she said. She then said that diesels were more likely to have failed air conditioning. They do at least have opening windows. She suggested there would be coaches at Basingstoke for the journey onwards from there.
The alternative was the 16.30 for Bournemouth. 2 x class 450 = 8 coaches. Very crowded as it had most of the passengers from the 16.25. I was in coach 2. If the aircon there was working it had absolutely no effect. No opening windows. It was a great relief to get out into the sunshine and 33 degrees on the platform at Basingstoke. No sign of any coaches, and the big information display on platforms 2 and 3 said the next train to Whitchurch was at 18.34, an hour later.
Maybe the aircon on all trains cannot cope with extreme heat and is liable to fail. Maybe that's why the two class 159 trains were only single units and the 16.30 was 8 coaches instead of a possible 12. But I would like to be told the truth about why there's overcrowding and a train is cancelled.