Null Magical Girl is a really weird and messy story that barely fits within the world of PMMM but I took a read through it to try and make some sense of it.
Trying to cut out everything besides the tiers stuff, the main quote is this.
I’m not proud of it, but I can’t even make an origami crane. If
you give me origami paper, all you’ll get back is a pile of
scrap paper. For me, the instructions are quite difficult.
“I’m not a fan of the way you do it,” Kyubey says matter-of-
factly. Even so, he encourages me to finish it.
The finished product is a three-dimensional object
composed of four equilateral triangles. In other words, it’s a
tetrahedron.
“What kind of railroad company would accept this as a
ticket?”
When I look closely at the tetrahedron, I notice a faint, blue
light leaking from it. Since the only material used was paper,
it’s odd that there’s light coming from it. The light gets
stronger and stronger as I look at it. The intense, blue light
illuminates the surroundings and dominates them in its
blueness. I notice the light penetrates the tetrahedron,
creating shadows inside. I’m not really sure if we can call
them shadows though. They aren’t projected onto a flat
surface like a normal shadow, but instead emerge as three-
dimensional objects.
The shadow forms a smaller tetrahedron inside the original
tetrahedron. Upon closer inspection, I can see even smaller
and smaller tetrahedrons inside it. Like a kaleidoscope,
layers and endless more layers of tetrahedrons manifest.
Inside each is an endless series of them. And outside,
infinitely many more emanate outward to surround it.
I, too, realize I’m in my own kaleidoscope. I remember the
sensation of falling. The scaffolding that had supported me
in this world suddenly disappeared from beneath me. I’m
filled with an indescribable anxiety.
Where am I falling to?
Into the tetrahedron, eternally downward into infinite layers.
From within endless deepness, a tetrahedron rises and
scatters to a limitlessly distant place.
From each point of my body, a tetrahedron evolves, both
infinitesimally small and boundlessly large, transfiguring me
into a singularity.
From far away in the labyrinth of tetrahedrons soaked in blue
light, I can hear a faint, high-pitched reverberation. The noise
gradually grows louder as a small dot penetrates the edge of
my vision. The sound and dot slowly expand in front of me.
Ah, I know what it is.
It’s a train, a steam locomotive that belches out black smoke
as its whistle cries out in its jet-black advance. The train
arrives with a deafening noise.
So, from this quote. It's pretty clear that the tetrahedron is some kind of a 'gateway' to this train station, also known as the Time Railroad. Eruna refers to the tetrahedron as the 'ticket' into the time railroad so to speak.
Once she properly folds it, she starts to fall through reality itself, as dictated by the 'scaffolding', and once she finishes she reaches the Time Railroad. This Railroad is used as a transportation method throughout all of time.
"That's right. Their magical power is without parallel. They
generate emotional energy almost passively. They’re using it
to colonize the entire galaxy—and if that isn’t enough, they’ve
conquered time as well.
“You know the Time Railroad, don't you?" Coeurl sighs.
I have no idea what that sigh’s supposed to mean.
"So, the railroad was built by Homo magica?"
"Yes. They’ve used the Time Railroad to establish their
eternal dominion from the end of the universe to the end of
time.”
Now, what is the Time Railroad exactly? It's kind of really vague and unclear, but from what we can get from the novel, it seems to be a system built by Homo magica (the evolved version of magical girls) that underlies all of time.
Homo magica have begun building a station of the Time
Railroad in a corner of the Ringworld, revealing its true
nature. It’s an ecosystem of organisms in a time loop.
All living things have the ability to travel back in time to
repeat evolution and change the timeline.
In an evolutionary time loop, predators use future information
to track their prey in the past, and prey struggle to avoid
future tragedies. Time and space are twisted by repeated
transformations of the timeline. Time travel is made possible
by connecting these points through a railroad.
So, all of these points in the timeline are connected through reality via the Time Railroad that Homo magica have built. This time railroad stretches along infinitely, with infinite paths into the future and the past.
In exchange for the time looping ability, the species
degenerated their intelligence, as there is no point in
learning from the past if you can endlessly reset upon a
mistake. Therein lies our opportunity. And thus, we overcome
many challenges to reach our goal—a station platform. From
the infinite future, the track stretches to the infinite past. On
the tracks, infinite trains with infinite routes are parked.
Note the "infinite trains with infinite routes", which is what I believe to be the author trying to express how the structure under the train can go through all of the infinite layers of reality. After all, this system underlies an infinite set of layers that underlie reality it seems, so it makes sense that a train that can travel through all of time could travel through every layer of reality.
At some point, Kosane (the main protagonist) undergoes an explosion that allows her to "fuse with the time railroad", and allows her to access every single point in time at once.
Who am I?
Kosane Kiriha. I am the human body indicated by that name,
that is my identity.
Ah yes, that was me. But not now. Not anymore. On the
platform of the Time Railroad, I was transformed with the
explosion of the time locomotive's boiler. I fused with several
Kyubeys and absorbed the locomotive and track itself.
Combining the temporal wisdom of the Time Railroad with the
spatial intuition of Kyubey's computational power, I escaped
the yoke of the three-dimensional world.
Here, it says she escapes the grasp of the three-dimensional world. Not necessarily High 1-B, but it's evidence that the time railroad is beyond our normal reality.
So it's my belief that the Time Railroad underlies all of reality as 'time' (or the 4th dimension so to speak), but the overall size of 'time' is much larger than 4-D in a way. As entering into the time railroad causes one to fall through an infinite amount of layers of reality. The time railroad is then noted to have an infinite amount of routes and infinite amount of trains, which is equivalent to the infinite layers required to get to the time railroad in the first place. This means there's an infinite amount of 'time layers' so to speak. Which can be construed as High 1-B.
This is all my interpretation though. I'd recommend you read Null Magical Girl yourself if you want an informed opinion, it's only 170 pages and has an English translation available with a short google. I don't have a strong opinion on the tiering itself, but I was just hoping that maybe my interpretation could provide some clarity.
