I'm going by my observations and a few chats with tube driver in the past, one which had worked on the line for 20 years. That said you could be right, not seen the air content of tubes stations analysed in any scientific detail.

I do know the Northern line in parts pre-dates electrification and deisel trains have been used as well as steam in the past -- last steam train was replaced in 1971 -- though which line I do not know and probably overground part of the service.

Though given it is underground then it is very probably a lot of dust and other crap air particles end up sinking into the tube system, certainly do not reccomend looking up in detail as can get dust from building waste and asbestos dust to some degree, though I'm sure nothing to worry about based up todays `safe` levels and limited samplerlings, Hmmmm.

http://en.wikipedia.org/wiki/Northern_line Interesting read.

https://en.wikipedia.org/wiki/London_Underground_diesel_loco... as proved insightful.

Think all we can really say is, it has added value air that some may not agree with more than others. Still least we all don't go around with surgical masks, I can see why some do and it will be those having the last laugh.