Modeling broad gauge in almost any common scale requires building just about every critical item for an operational model railroad from scratch - locomotive mechanisms, track, trucks and wheel sets. That's why most folks going after an unusual prototype gauge will pick a functional gauge, and actually adapt the scale to suit the gauge.
The best example of this practice is G. The G gauge is the common track gauge for a number of different scales to represent various prototype gauges using the same model gauge. At 1/32, G gauge is correct for standard gauge track. LBG used 1/22.5 on the same track to represent European meter gauge. And a sizeable contingent (including Bachmann) uses 1/20.3 to use G track to correctly represent 3ft gauge. Interestingly enough, G track would be just about spot on for your 7ft broad gauge in O (1/4") scale.
The closest bigger track gauge to N with any commercial support is HOn3 at 10.5mm gauge. At 1/160 scale, 10.5mm represents 5ft 6" gauge quite accurately. The HOn3 wheels would probably not be too grossly oversize for N scale broad gauge, so this might be a workable combination. I don't see any other viable combinations in N scale. Sn3 track, the next commercial gauge up, would be 90" or 7.5ft. And the Sn3 wheel sets would be huge in N scale.
my thoughts, your choices
