Stephen, the taper in the stack is essential to the Lempor exhaust. Again...you need to remember that we are a museum with the mission of historical accuracy and the Lempor exhaust would not only not be correct....but would ruin the sound of the locomotive. Lempor exhausts are almost silent and you would not have the spine chilling exhaust note as the train climbed the grade headed to Alna Center...........
Actually, Lempor exhausts are quite noisy, depending on the amount of work the loco is doing. The exhaust has a different sound - generally a crisp, higher pitched, rasping note. When Boston Lodge rebuilt Garratt No. 87 a Lempor was fitted and if you watch - and listen to- several YouTube videos you'll get the idea! The other Garratts have a deeper, more muffled exhaust.
Several of the UK heritage railways have locos fitted with Lempor (and similar) exhaust systems - lots of info here (
http://www.martynbane.co.uk/) for anyone interested. The Mount Washington Cog work was carried out by an Englishman, Nigel Day.
The Festiniog's "Linda" and "Blanche" have had Lempor and Kylchap exhausts respectively, but both have now been removed and a plain blastpipe with petticoat refitted. The nozzles are prone to carbonisation - when cleaned out the loco would go like h*ll but not steam, when a bit bunged up they'd steam like a witch but be very slow to accelerate. I think the main reason for reversion to the simpler, original arrangement was down to the fabricated nozzles and diffusers being life expired. For all you fans of little engines working hard watch this:
http://www.youtube.com/watch?v=Acsz688-ohwHTH,
Nick