I think part of the problem is the genuine lack of wideranging prototypes - most USA NG was gone before it hd a chance to dieselise, or hung onto steam long enough to turn straight into a steam-for-pleasure operation. Remember also that Bachmann got badly bit with their diesel - a large proportion of the production run went to the discount warehouses, and a second run will probably never happen.
From memory, I can think of the following 3ft gauge 'mainline' unit types
1. White Pass #81(2nd) - one-off variable gauge Alco, ex-US Army #3000
http://narrowmind.railfan.net/WPYR/diesel/81_diesel/81-wpyr3.JPG < Link To http://narrowmind.railfan...81_diesel/81-wpyr3.JPG
2. White Pass GE shovel nose Co-Co - can't remember the designation but one line only
3. White Pass Alco D535B - as far as I can remember, they were the only US customer for these - and while far from perfect, LGB's effort limits the market size for anyone else
4. White Pass MLW DL535E - 4 built, one on the WP, one written off, two with US gypsum I think
5. Georgetown loop switchers 130&140 - U4/5or6B's I think, ex US Gypsum, definitely switcher outline, there's talk of a brass model anyway
6. another US army owned units that ran on colorado lines in the late forties, #4700N
http://narrowmind.railfan.net/NG_Diesels/USA4700NDPL.jpg < Link To http://narrowmind.railfan...00NDPL.jpg Been looking for plans of those for years.
7. GE 70 tonner on C-C trucks for US potash
http://narrowmind.railfan.net/NG_Diesels/GE_70T_CC_36in_US.JPG < Link To http://narrowmind.railfan.../GE_70T_CC_36in_US.JPG
8. SP #1, GE50 tonner B-B.
Only the first 2,3,4 really meet your criteria I think, plus maybe 1 and 6.
Apart from that, what have I missed? Most non US users of export type US diesels have been metre (northern and east africa, india, asia and much of south america) or cape gauge (newfoundland, southern africa, australia), so are more logically modelled in 1/22.5 or 1/24 anyway - 3ft is used on some south american lines, columbia, peru and cuban I think, but that's it.
J.