Part of the problem is that much of the London tube is quite deep underground. New trains on the sub-surface lines have air conditioning, but on the deeper line

Part of the problem is that much of the London tube is quite deep underground. New trains on the sub-surface lines have air conditioning, but on the deeper lines it's difficult to extract the waste heat. It can be a problem in summer. I've seen a couple of passengers faint from the heat on hot days.