There are several built-in vehicles and signs in this mod. For many people, it's enough, but for some special users, such as servers, open saves, or realistic projects, it's usually not enough.
For solving this, MTR supports custom resource packs. Right now, it supports to add new vehicles and signs. Besides, you can also moderate something like decoration blocks in vanilla way.
Please note, the majority of resource packs are not currently compatible with the 4.0.0 preview builds. Unless they have been marked otherwise by the author, assume they will only work in MTR3 until 4.0.0 fully releases!
Basically, a Resource pack is a zip-file which, if activated, can change or add new textures, models, music, sounds, languages, texts, without editing Minecraft, or mods directly.
In the context of the mod, Resource packs can provide new sign textures, new trains textures and/or models and/or sounds.
Most resource packs are packed in ZIP format. You can just put it into the .minecraft/resourcepacks folder, open the game, enable it, and use it like using the built in MTR content.
Some resource packs aren't packed in this format. For these cases, you should open the file and find if there are ZIP files in it. If you directly find pack.mcmeta in pack, you should decompress it into a folder in your resourcepacks folder.
For other questions about the use of resourcepacks, see Minecraft Wiki for more information.
For creating custom trains and signage, see Making a Resource Pack.
For importing custom train running sound from BVE4/OpenBVE to an existing MTR custom train, see Custom BVE Train Sound.
You can find Resource Packs on the MTR Addons Site. There is also a dedicated channel in the MTR Discord server, deprecated after the full release of the MTR Addons Site, but ensures legacy 3.0.0 content remains available. Some packs have a page on this wiki, they are listed down below: