25 years ago when i built mine, 30" - 36" was the norm. i built mine at 42"on the main level and the upper level gets as much as 8" higher than that. If i were to do it over again my main level would be at least 50". Why? My old joints and aftermarket knees don't bend like they used to, plus i like viewing trains from eye level (a habit i developed as a kid lying on the floor watching trains run on the carpet).
Kids can stand on stools, or, come back in a few months when they've grown taller.
It's your layout do what makes you happy.
jackson
