@DJ Dangerous
12mm gauge track is readily available from
Peco, Shinohara, Bemo, (All HOm. )and now Hornby (TT 1:120), there are others Tomix (TT 1:120 Tram style inset in roadway segments ) etc.
00n3 is starting to be talked about again, Rolling stock kits from Roxey and Branchlines for IOM, Loco's from Branchlines.
Irish rolling stock where do you start Parkside Dundas (and some members only from the 009 society) plastic kits for the T&D
Plenty of Brass scratch aid stuff From Worsley Works covering a vast range of lines for locos and carriages, With the old Backwoods Miniatures ranges and Nineline's ranges having now been taken over and reintroduced by Neville Kent NDrive who intends to provide some RTR mechanisms as well as the kits. Other mech's can come from Mark Clark Loco's N Stuff Primarily a 7mm provider (But does do things in 12mm gauge)
Halling Motor bogies can always be hidden by skirts on a tram loco or frames on a diesel. (NOTE LINK IS TO MY FAVOURED SUPPLIER OF THESE UNITS, OTHERS ARE AVAILABLE).
Also with the advent of 3D printing there is a myriad of other stuff out there.
If looking for vintage kits then Southwold, Rye and Camber and other systems were also covered.
Take a trip over to 00n3modellers@groups.io
Colin replied while I was typing.