RE: Gn15 4 wheel mine cars
You may want to look at some OO scale British outline hopper wagons by Hornby or Bachmann Branchlines. Cut the buffers off and add either link & pin or knuckle couplers. Paint to your liking, and you will have some fairly credible two-axle narrow gauge industrial hoppers for Gn15. The cars are already gauged for HO.
Just a thought,