OK so I'm one of the (Oxford Rail) N7 fans.
I pretty much have to be, as I am working on this layout project.
Fortunately I already was a fan as I currently live in Homerton (on the east side of Hackney in London) not far from the the Great Eastern Railway line on its viaduct running up through Cambridge Heath, London Fields and Hackney Downs. This was one of the chief routes on which the N7 was used and a little bit of reading got me interested in the whole history of the GER, the only 'full size' railway company to concentrate all its loco and rolling stock building and all serious maintenance in London. The N7 is both quite an interesting loco in its own right and very much a representative of the railway that built it.
Anyway enough of that. This post is an adjunct to the long running thread on Oxford Rail's model, and just records that it is possible to take off the rather plump wheels that come with it, mount them on longer 2mm axles you may have spare, gauge to a 16.5mm back to back, and (after a fair amount of filing) things look like they should work.
The filing is needed because the wheels are going on for 3mm thick. The low footplate means the top of each wheel protrudes through it, so clearance is needed between the wheel face and the footplate and as it happens also at the sides to clear the flange. The front driving wheels are almost OK when taken out to EM. The splashers have plenty of room inside but a little has to come off the sides of the opening in the footplate.
The second driving axle is fine as long as all the sideplay allowed for sharp curves is taken out. There's around 2mm in spacing washers behind every driving wheel -- I have to do some trials out on the line to see what this means in practice.
For the rear drivers, a lot more filing time is needed. Oxford's designers have made the water tanks solid metal (which is brilliant for adhesion weight) but the allowance for the wheels to project into them isn't wide enough for the wide wheels taken out to EM. I didn't do a brilliantly tidy job of making more space, but that is all that will need doing. It's just that it takes a while; I don't really know why the clearance is tighter on the rear drivers than on the others but that's the way it comes. As with the front wheels a little room is needed for the flanges but mostly it is the wheel face that needs more room. The axles can rock slightly so extra space must be allowed otherwise they will rub and also connect the metalwork of the footplate to one or other of the rails. In extreme cases both rails might be connected; you will soon find out as this will short out the track power.
Other than that, apart from my determined efforts to ruin all the detail parts, I have done nothing except to remove the reversing lever and the condensing apparatus lever from the left hand side of the boiler moulding. And taken the lions with their unicycles off in preparation for a later-style British Railways emblem. So there are few more steps to get back to an accurate model.