I'm looking for a lightweight 16" bike for commuting by train. After arriving at the station in Paris, it's a 2 or 4 km ride depending on my destination.

I have a Tern Verge X11 which is phenomenal. I have ridden it down the Champs Elysées and all over Paris.

The problem is the train.
There are very few places for bikes on board.
When there are few people, like today (long holiday weekend), no problem. I can find a seat and leave my Tern unfolded.
But on a normal day, the train can be crowded (not enough seats). Or even overcrowded: All it takes is for a rush hour train to be canceled for whatever reason, and passengers are packed in like sardines for 20 (one stop) or 40 minutes (final destination).

I left a very old 26" MTB (no suspension) in the bike parking of the station, but it's not optimal : it's a tank compared to the racing X11 (no fun riding it) and it's a break in my trip.

I need a more compact folding bike than the X11. Let's be honest, there is only Brompton.

The Tern BYB is not as compact and I don't like the idea of a double fork.
The new Decathlon Btwin Fold Light 1 s seems interesting but opinions are mixed. If they can fix the problems, it's a cheaper alternative (€850) although less compact (71 x 69.5 x 35 cm).
I like the Helix, but the company seems to be in a rut. It's less compact but the width is great (and key on a train).

I also need a light bike because I have to carry it up and down some stairs at the stations and go through a turnstile.

So I went to the Brompton Junction in Paris during my lunch break. With my Tern of course.
I tested the P line (mid and low bar). Same weight as the X11. I find the C line too heavy (13 kg).
The environment was ideal: flat street. It rides well (I prefer the mid) and with its compactness and weight it would be useful.
However, I was disappointed by the transmission and, to a lesser extent, the brakes.
I lifted the T line and, oh boy, it looked like it weighed a feather. It would be very handy on the train and in the stations.

I ruled out the titanium Brompton clones because there's no easy way to know their reliability.
Also, shipping and VAT on price + shipping (a sure thing now) makes the cost too high (and too close to Brompton's).

The T line would be great, but it's very, very expensive.
At that price, I would expect better gears and brakes, even if heavier.
The P line would be more reasonable.

I understand why some people have built their own titanium clones.
I wish I could do the same.