With deepest thanks to the fruits of a long-ago correspondence with British railway expert Michael R. Bailey, I thought I'd set out the basic facts, should anyone -- Robert Grenier or others -- actually have the chance to recover either engine. As it happens, both were of the "Planet" type originally developed by Robert Stephenson & Co. As Michael describes it, "HMS 'Terror' had a four-coupled (0-4-0) version (strictly a 'Samson' type) built by Robert Stephenson & Co., which was used on the London & Birmingham Railway as a contractor's engine from about 1835 until sold to the Royal Navy in 1845." HMS "Erebus," on the other hand, was fitted with a 2-2-0 "Planet" (see illustration above), but not one built by Stephenson; it was rather a copy produced by the firm of Marshalls of Wednesbury for the London & Greenwich Railway in 1836. Both engines had seen about a decade of service, and were in a sense being "retired" in order to reduce older rolling stock. While they were of closely similar types, the array of the axles, as well as (Michael's note) "the variation in the height of the axleboxes" would enable them to be readily distinguished.
But what sort of shape would these engines be in after more than 160 years? Michael, whose experience is founded upon his work with "Operation Iron Horse," which involved recovering engines from an 1857 shipwreck off the coast of the Hebrides, is confident:
"The components on the 'Terror' and 'Erebus' should all be in good condition in the absence of oxygen. Wrought iron lasts well, unlike steel. The non-ferrous components should be largely unaffected. Their timber outer frames were clad on both sides by wrought iron plates. The plates should still be ok, but the timber may have gone. The condition of the ships' and locomotives will, of course, be quite dependent upon the prevailing sea conditions...... You will know well I'm sure, that iron-work etc. attracts marine life which attaches itself and builds up over the years and can distort the appearance. The main difficulty with the 'Iron Horse' expedition has been that all components have been 'buried' in a large lump of concretion (concreted crustations) and would be quite un-recognisable toa sonar scanner. The divers have had to chip away the concretion with pick to release each component, which is why it has all taken so long. However, it is likely that the crustations have been present because Scotland is on t he receiving end of the 'Gulf-stream' and is thus 'warm-water', and hence very different from King William Island."
So far, the engines have eluded all attempts at finding them. David Woodman spent a great deal of time and energy hauling a magnetometer over the seasonal ice in areas where Inuit testimony indicated one of the ships had been anchored, in hopes that either engine would produce a notable magnetic "signature" -- but all of the the targets he identified turned out to be large rocks and other natural phenomena. I don't know whether Robert Grenier has or will employ similar techniques, but I do hope that, if he does find something, he's able to access the expertise of someone like Michael Bailey.