This link is about M2 locos, which inagurated the 1st Journeys of "Udarata Menike" & "Yaldevi"
http://www.kodikara.net/blog/2011/07/24/m2-%E2%80%93-the-legendary-workhorse-of-the-sri-lankan-railway/
I agree with you, people make money selling N2.( 4 times more than normal air) I am also using N2 and I feel that presure dropping rate is less compared to normal air.