I use https://mods.factorio.com/mod/TrainBigCarriage with https://mods.factorio.com/mod/TrainSpeeds to make my trains shorter. Would it be possible to apply the 4x storage if his mod is found (maybe with data-updates.lua)?

This mod is intended to be balanced with vanilla trains stats and vanilla combat vehicles.

And there are simply too many mods trying to apply their own set of train stats (cargo slots, weight, power, acceleration/deceleration, speed, health, etc) and on different data stages.
Adapting to one or two mods sounds easy. Yet this would quickly become very difficult (if not impossible) to re-balance, when multiple mods want to do the same thing. Therefore, sorry, I cannot include such changes as formal release.

However, you can modify your own copy to what you want to achieve.
Decompress the mod file, then open prototypes/armoured-trains.lua with a decent text editor. Locate ["armoured-cargo-wagon"] and inventory_size = 20,. Then change it to any value you want, and save the file.
[https://mods.factorio.com/mod/TrainSpeeds] seems to tinker with other stats like weight and max power. You may have to adjust those lines accordingly.
PS: A decent text editor means anything except Microsoft Notepad... If you do not have one yet, try Sublime Text or Notepad++. Both are free to use.

Ah, that makes sense. I inspected TrainBigCarriage and it seems it's much easier to modify that so your prototypes get higher inventory size from there. I just need to add your mod name as an optional dependency. Thanks for the explanation!

P.s. flo's notepad2 is pretty good :) And I am a PHP website developer so I understand the concept of (optional) dependencies.

EDIT: https://i2.paste.pics/DN82C.png seems to work, I think it's supposed to have only half the inventory size of the vanilla wagon?

Ah, that makes sense. I inspected TrainBigCarriage and it seems it's much easier to modify that so your prototypes get higher inventory size from there. I just need to add your mod name as an optional dependency. Thanks for the explanation!

P.s. flo's notepad2 is pretty good :) And I am a PHP website developer so I understand the concept of (optional) dependencies.

EDIT: https://i2.paste.pics/DN86F.png https://i2.paste.pics/DN82C.png seems to work, I think it's supposed to have only half the inventory size of the vanilla wagon?

Nice, glad it works, as this would be much better than integrating every mod re-balance changes into my own mod.

Cool you are a developer.
I have seen some users reporting a game crash on start. End up it was caused by file corrupt, due to usage of M$ Notepad. So when I comment on text editing, I just assume the users/readers do not know first.
flo's notepad2t looks simple and clean enough. But I see the last version was nine years ago... Is it a bit too old?

Yes, supposed to be exactly half the inventory size of the vanilla wagon. Lore-wise, the reduced space is used by extra armour (increased resistances and health) and grid equipment.

There is also https://github.com/XhmikosR/notepad2-mod , which is a bit newer and has a few more extra's. And there is also https://github.com/zufuliu/notepad2, which is essentially a fork of a fork. Or even https://github.com/rizonesoft/Notepad3. I just use flo's original, since I only use notepad to do minor edits to code when I'm playing games :)

There is also https://github.com/XhmikosR/notepad2-mod , which is a bit newer and has a few more extra's. And there is also https://github.com/zufuliu/notepad2, which is essentially a fork of a fork. Or even https://github.com/rizonesoft/Notepad3. I just use flo's original, since I only use notepad to do minor edits to code when I'm playing games :) Software age for something as simple as notepad isn't an issue for me.

I guess I tend to need more heavy-duty text editors. When I work on mods, for example, I need to work on multiple files at the same time (say, library file, main script file, locale files), and very often for multiple days. So a text editor that can remember my last open state (opened files, as well as cursor position) and auto-reopen files would be a charm. Sublime Text can do this perfectly for me. In addition, its "Quick Find All" (which select all matching instances without entering any pop-up dialogue) is super useful to me, like renaming a function to the whole file in just a second.

There are still times for lighter duty though, like editing a batch file or configuration file. I will try some newer ones among your suggestions. Cheers!