T.I.D. Is thought to stand for "Tug Invasion Duty", the first few were built coal fired and then on, oill, with the coal fired ones being converted to oil later. this seems to be the general consensus. Also after the war many were converted and I have been told that there is one in the Mersey now that has a very strange wheelhouse with the funnel sticking out of it. Dunno how true that is? :)