Boy I have to comment on this one! Some of my ancestors are from the IOM, so I've visited and ridden the various railways there. Over there the Eureka would look a bit like a Big Boy on the D&S. When I first looked over the locomotive on our trip (the #13, sometimes called the 12A) I was struck by the coal compartment that looked like the volume of a couple of suitcases. Also, couplers would be a problem.
The IOM railways are truly wonderful - if anyone here gets a chance to visit the island, It is a beautiful place with tons of history. As you ride the Snaefel Mountain Railway, which is electric, consider that it was built at around the same time that Otto Mears was planning to build the Ouray and Ironton Electric Railway to connect the Silverton RR with the D&RG at Ouray by going down what is now the Million Dollar Highway (US 550).
Ironically, as I look out my window here outside Ridgway, Colorado, the distinctive mountain is Mt. Sneffels - interesting for a descendent of a Manxman, where the highest point is Mt. Snaefel.
If you are a rail buff, it would be a good idea to avoid the crowds by avoiding the IOM during TT week (unless you're also a race fan), but again, very highly recommended!