Now, I first started realizing the existence of folding bikes when I moved to Japan and saw all these manly men riding bikes that might as be built for a 5 year old. Only later, after much critical thinking, that I finally googled it and found out that they're in fact not toys at all but actually foldable bikes. So, besieged by dreams of travelling by train to rural areas and then happily tottering around on my bike, I went ahead and bought (a shimano, I think?) cheap folder for $240.

Now herein lies the problem: It's heavy. According to the sales person, it weighs 15kg, and I think I nearly fractured a hip just by walking a few step with it slung over my shoulders. I really can't see myself struggling with folding it, putting it into a bag, then lugging it up the multiple stairs to the platform. Not without a considerable amount of effort, at least.

I'm not sure what exactly I'm asking - but is there any solution to make it... more portable? A better bag, or a folding bike bootcamp, or a membership to a gym?

Thing is there are MANY types of folding bikes and each does a different job, Some are much easier to lug around when doing muti modal travelling and some are better ride experiences but not that fun to lug around. The more expensive ones weight much less thatn 15kg and probably somehow incorporate some special folding technique that you can still roll it around after you folded the bike instead of just "slinging" it over your shoulder. I'm not sure if these models are available but if you are of a average size under 6 feet you can check out in your area of japan for a strida 5, bike friday tikit, any brompton. These three folders i've mentioned can be easily rolled along the floor and are relatively lighter than 15kg.

can you take a pic of you bike folded and unfolded? maybe there's a way you can roll it around.

What do all the guys in Japan carry up the stairs to the platform? Get one of those.

I haven't really seen any guys carrying a bike into the train - just saw them riding on it in the streets.

It's a bit too late for me to buy a lighter one - I inquired about it, and they seem to cost around $400, which I certainly cannot spare right now. Okay. 14.1kg. Sigh. As if 1kg would make a difference.

It's not even a Shimano, my bad. Seems like it's a relatively unknown brand. https://yutjapanbike.igetweb.com/catalog/p_31219.jpg

https://deemkie.com/wp-content/upload...be-bicycle.jpg
It folds up similar to that. Certainly a bit awkward to wheel unless I pull the saddle in all the way down, and even so I can only wheel it forward, trying both ends with a rope to keep it together. Perhaps I will. But it seems that in Japan they require bikes to be carried in bags - I'm thinking of simply covering it with a bag, exposing the wheel, and wheeling it. Don't know how viable it is unless I try it.

That looks to me like a rebadge Dahon Curve, and without knowing much about the specs, you might have got a bargain. I suggest not folding it til you need to. I lift my Downtube by the seatpost, only bag it when on the platform, and don't even do that unless it's absolutely necessary.

I agree with sammyboy, I always roll unfolded. Lift only when needed, find stations with elevators and escalators.
I've seen a guy with a Dahon boardwalk S1 rolling it up the escalator out from Penn Sta during rush hour.