Such would be nice for when playing with trains. A nearly bottomless 6x6 container for single item type, would it be possible?

The actual Warehouses from the other mod are propably still a bit UPS unfriendly. There doesn't need to be a thousand item slots for game to manage, so i'm going for performance benefits with the idea as well as the fun aspect.

I have not done much development recently, but one thing I had been working on was a 2x2 chest, mainly for more loader access, though it would probably help with train loading/unloading. I will probably not make a huge one, 4x4 or 6x6, though.

I tend to use a mod that gives lots of inserter pickup/put down options, and using this, can put 6 or even 7 inserters per car accessing the one normal sized chest, in case that idea helps.

I would actually love a 27x1 chest - 4 train wagons long - and a 7x1 - two cargo wagons long.

That is unlikely to happen, in my opinion, it would be too powerful. The limitations of a small chest with only a single item being stored are meant to offset the insane power of unlimited storage.

For unloading a train, using larger chests to buffer and then unloading those into the compression chest for longer term and compact storage tends to work well.

But in megabase scale you have lots of belts for say iron.. try 40 belts? If you want to buffer even some of that through a single chest you can only do that from 2 sides, for example 2 loaders in, 2 out. And then you would need several compression chests, which would in storage space be quite rediculous.

My current solution to it is Merging Chests (WideChests) mod https://mods.factorio.com/mods/Atria/WideChests .
It's not infinite but it's got high performance and you can use it for trains.

Mega bases are why I started looking into making a 2x2 chest, for more access points, but hopefully still small enough to not break the game.

In general, I think the compression chest shines best when collecting secondary or waste products. If I recall correctly, the first time I used this mod was for collecting stone and coal when I just wanted to clear resource patches to expand my base. Both of these are used throughout the game, so collecting them is worth while but they are used slowly so limited access to the chest is not a problem.

A large one item bufer would only be useful fo train end points that ONLY HANDLE ONE ITEM. This is not powerfull. If anything it needs "more power" (i.e. multiple stacks to unload and load between). I right now use merge chest mod with steel chests and I store a quarter million ore / iron there. Using LTN that is enough buffer until new arrives.

What it would do - possibly - is allow performance ;) Large chests i have been told are UPS unfriendly. This - little stacks "visible" - MAY work around this.