The figure I was thinking of, was four. 3 feet : most railways -- electric, and "not" -- and tram routes. 3ft. 6in. : Snaefell Mountain Railway. 1ft. 11-and-a-half in. : Groudle Glen Railway. 4 feet 8-and-a-half-in. : Douglas Southern Electric Tramway (Douglas -- Port Soderick via coastal route: ceased running on outbreak of war in 1939, and never reopened).
@xotGD, your figure agrees with mine: so it would seem proper that you take the floor. If there's anything legitimate on one or more other gauges, of which I've been unaware (I'm not thinking "pier miniature railways" or such stuff), maybe that could be discussed.