Modlist:

http://prntscr.com/115b6a1

I suspect it is some kind of incompatibility with the rendering that the Optifine causes.
Is there a solution other than uninstall it?

Some screnshots:

http://prntscr.com/115b8j5

http://prntscr.com/115b97b

I have tried out the mod in 1.16.5 and in 1.12.2 both either with optifine installed and the cache folder deleted and without any optifine, but I am still not able to see the trains in the inventory, which makes the navigation through the trains very laborious.

Clear your cache folder with fix it, cache folder is in the instance of Minecraft your using.

Hi, Lasse here, on a second account.
So the cache folder I've deleted was in .minecraft

But I've actually fixed the problem. As I was unintentionally browsing in the settings, I clicked randomly on "shaders" which then said "set Antialiasing on 0" and then I thought, that Immersive Railways may have the same problem with the Antialiasing, so that I set it on 0 and deleted the cache folder again. Then after starting minecraft again, it worked and now there is every icon in the Inventory.

1. Disable Optifine fast render.
2. Delete the cache folder in your .minecraft directory
3. Startup minecraft.

That didnt work for me
Mods:
https://prnt.sc/19h0i9z

How it looks for me :
https://prnt.sc/19h0m1m

1. Disable Optifine fast render.
2. Delete the cache folder in your .minecraft directory
3. Startup minecraft.

Thanks its working.