Late Monday afternoon. Wandered down to Marble Arch to catch a train out to Stratford.
La la la... through the ticket barrier. La la la... down the escalator. La la la... along the passage way and down the stairs. La la la onto the platform.
It was as though I had walked into a brick wall. It felt really hot. It wasn't particularly hot outside, or anywhere elso on the station, just on the platforms. I acclimatised fairly quickly, but the contrast in air temperatures over the space of a few feet was incredible.
I seem to recall that the Evening Standard ran some articles a few years ago about the differences in ambient air temperatures on different London Underground lines - my recollection is that the Central Line was the warmest because of the type of braking system that the trains use.
Anyhow, if my experience of this week is anything to go by... Need a sauna? Don't spend big bucks on a fitness club or gymn. Don't fly off to Scandinavia to experience the real deal. Just buy an Oystercard and check out the platforms at Marble Arch. Delightful.