Getting your kit off for charity is pretty mainstream these days thanks to the ground-breaking Calendar Girls, but this was nature in the rawest form - no strategic cupcakes or rugby paraphernalia here to preserve the modesty of participants* (and some interesting gravitational effects too).
So why do it? In this case it was in the highly laudable cause of helping to buy two digital mammography machines - a snip at £750,000. People came from all over the UK to take part, so many they had to be sent round in 3 batches in the 40-seat train.
Now, I don't pretend to understand the economics involved in the purchase of this sort of equipment, but I do know that, in many cases, hospitals and schools pay well over the odds for equipment that could be sourced more cheaply elsewhere.
By the time it has passed through the Byzantine workings of purchasing departments and agencies, a straightforward transaction can accumulate a substantial weight of surcharges and premiums to cover the manpower costs - and that's without the action of market forces.
I sincerely hope that the nude roller-coaster riders of Southend will get full value for their £22,000 and didn't bare their all to pay the wages of some administrative middleman in a purchasing department somewhere.
*This is amply demonstrated elsewhere: the Metro performs what it obviously feels to be a vital public service by publishing an extensive photo gallery of the event.