I genuinely don't see the point of the 'Superlight'.
I get the principle of a lighter bike being better (both riding it and carrying it), but in this case, I don't think it makes enough of a difference to justify the additional price.
My commute also involves an overground to London and I wheel mine along the platforms and only fold when I'm at the train door. The exception being that at my outlying station, I have to take it up and over a footbridge, so I hang the seat on my shoulder and carry it that way.
I think I'm supposed to fold it and carry it, but no one's stopped me in over four years.
I don't think it's the weight that's necessarily the problem, but how it forces you to carry it. When it's folded you have to walk with your arm out to the side ever so slightly. This is what causes the fatigue in my experience. When I take it down the stairs, I leave it unfolded and carry it by the maintube. It's so much better balanced and easier to carry that way.