Wheelbases shorter than the track gauge are generally not very reliable without additional balance wheels front and back. The physics make it unstable and make tracking around curves difficult.
It was not unheard of for some four-coupled locomotives with pilot and/or trailing wheels to have close-spaced drivers. Some of the Porter four-coupled locomotives had wheelbases as short as 36" and gauges offered wider than this. I would suspect few were sold in this configuration.
It is also difficult to accomplish with wheels of any significant size and since driving wheels are generally relatively large to provide a higher effective speed for each revolution they are rarely spaced this close.
Looking at the basic mathematics, assuming 15" gauge track (this is the Gnatterbox), with a 12" wheelbase you could feasibly use driving wheels with flanges about 11 inches in diameter, allowing for some clearance and mounting brakes on the outside of the wheels rather than between the wheels. Wheels with 11" flanges have a running (driving) surface about 8.75" in diameter, which means to achieve each mile per hour of speed the wheel needs to rotate at about 39 RPM. Typical industrial locomotives move at 4 mph in many cases, so the wheels need to turn at 156 RPM. A Lister CS engine ca. 1958 generates 8HP at 850 RPM, which means you can only gain an advantage of 5.5x through gearing, which results in a locomotive with almost no hauling power at full throttle speed of 4 MPH.
Small diameter wheels means low maximum speed and lower hauling capacity relative to the available horsepower (due to less gearing advantage being available). Note that this isn't a linear relationship. Wheels can be too large for an engine as well (due to inertial concerns).
On the other hand it has been done. I've seen several photos of several early or home-built locomotives with pilot trucks that had wheelbases shorter than their gauges. These trucks usually had wheels in the 9 inch to 12 inch diameter range with a wheelbase of approximately 24 inches for a gauge of 36" or standard gauge.
You can also see examples on many old narrow gauge Plymouth DLC6 or similar gas mechanical locomotives that have been rebuilt for standard gauge. Have a look through the Plymouth sections of the following website:
Good luck with your build, but you will probably find the locomotives and/or cars rock back and forth and don't track very well. Problems in real life are often magnified in the small scale.