>>>> [one can] build a train that gets a structural failure if greater than
>>>> N passengers of a given mass (I assumed the weight is due to
>>>> electromagnetism or that gravity doesn't affect time for simplicity)
>>>> are on board.
>>>
>>> Prove it.  You seem to think it's obvious, so the proof should be easy.
>>
>> By experience we know this is so.
>
> What is your experience with arbitrarily stretchable trains?  I don't
> have any such experience.
>
> Prove that you can build a train which can be stretched the way you have
> proposed and yet which will fall apart if one more person steps foot into
> it.

As I stated previously, when I looked this argument I used a train
(platform) positioned along the y-axis.
I let the length of this platform be 100 light-years.  One end is at
x=0,y=0, and the other end is at x=0,y=100 light-years.  I let the
passengers be spaced at one meter intervals.  They are boarding one
end of the train at 1 person per second and are leaving the train at
one person per second as measured by the passenger's watches at each
end of the train.  There are N people on this train in N chairs
intially (when all points of the train are on the y-axis).  I let
there be X atoms on the platform capable of supporting these N people.

 See also

https://home.deds.nl/~dvdm/dirk/Physics/Fumbles/Everyday.html