OpenTTD allows the use of NewGRFs and different base sets for graphics, sounds and music.
These files require special formats and you may need to use the following helper programs to assist with the creation of these add-ons:
- nml High-level language and compiler for writing NewGRFs. See also the specs and tutorial
- grfcodec Compiles NewGRF's low-level programming language NFO into a GRF file. Use the enclosed nforenum to check the code for sanity. See also the specs and tutorial.
- catcodec Compiles the sound set description file (sfo) and the sounds into a sound set
For further help see the wiki, the DevZone or visit the forums.
NewGRFs can be made available to players via the online content.