Piston and valve rings.

I'll hazard a slight addition to @Jamessquared's guess about an overfilled boiler, which seems very plausible. Having previously seen 70000 sit in Ropley yard with a very noticeable stream of steam from the draincocks, I'd guess the multiple valve regulator doesn't close as well as it should. So for backing down on the train, the driver has the drain cocks open to avoid a build up of pressure in the cylinders as he tries to come smoothly to a stop, and same when sitting in the platform. This should of course keep the cylinders warm before starting from the platform, but if the boiler is overfilled...

 

I had meant to mention that aspect. As that train was starting off, the driver can't have seen anything in front except a huge white cloud.

 

That has to be the one thing to be said in favour of tender-first running!

 

Always wondered whether the direction drain cocks empty is ideal...

 

That begs a question you might ask on the Isle of Man thread!

 

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I've seen locos with drain cocks pointing at right angles to the track. Not the best of ideas, especially when you consider that the exhausting steam can be fairly powerful. I've often wondered why they couldn't exhaust up the chimney but the necessary drain holes would probably mean as much is vented out of these as up the chimney.