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.
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.
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.
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.
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.
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.