Dear Gn15 crew,
If memory serves,
wasn't it the doyen of 15"gauge railways, one Sir A. Heywood, who set out the "3 X gauge" rule for body width,
because 3 X 15" gauge = 45" - 48" over body approx max,
which is just wide enough for a person to be seated,
and permit other person to walk past them down the "side aisle" of a passenger carriage,
without being "unduly unstable"
(Or am I just mistaking Urban Legend as Fact again?