It says it right there on the tin, An addon/resource pack for Immersive railroading that adds the Steam Locomotives and various rolling stock from the video game, Red Dead Redemption 2 and the online version of the game/
Trains such as these.
This is probably going to involve ripping the models of the actual trains from the game and using them as reference material when modeling the trains for the resource pack/addon. Something my crummy laptop is incapable of doing without my graphics card burning up and bricking my computer.
I'm not saying all of them, but some pieces of rolling stock share the same model but have different textures of the different liveries of the vaious railroad companies in the game. same for the locomotives, of which there's only two models that also have various skins representing the livery of different companies ingame. However, some rolling stock with interiors wont share the same interrior design and style.