The prototype standard is indeed 6' between adjacent lines on double track (measured from rail to nearest rail, not between track centres). This increases to 10' between adjacent pairs of running lines (Up fast/down fast//up slow/down slow) or between running lines and an adjacent loop or siding (up loop//up main/down main//down loop).
However, as others have stated, this all becomes a bit academic when model railway curves and overhangs are brought into the equation.
However, as others have stated, this all becomes a bit academic when model railway curves and overhangs are brought into the equation.
