.NML File Extension
NewGRF Meta Language File
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An NML file contains NewGRF Meta Language (NML), which can be compiled into NewGRF .GRF resource files for the OpenTTD transport simulation game. It stores plain text in the NML syntax, which consists of blocks of text. NML files are used to create or modify resources used in the game, such as trains, aircraft, bridges, airports, ships, buildings, industries, and sound effects.
More Information
You will most likely only encounter an NML file if you are looking to create or modify content in OpenTTD, which is an open source transport simulation game based on Transport Tycoon Deluxe. The NML file is typically accompanied by .PNG, .WAV, and .LNG files that are part of a project that defines the properties of the OpenTTD resource and what it looks and sounds like.
NOTE: The original version of GRF files only supported simple sprites, but NewGRF GRF packages compiled from NML files and other project files enable OpenTTD users to apply more custom and advanced changes to the game.
How to open an NML file
You can open an NML file with any text editor since the contents of the file are saved in plain text. You can use Microsoft Notepad (Windows) or Apple TextEdit (macOS), which come bundled with their respective operating systems, to open and edit NML files.
Some source code editors provide helpful features, such as syntax highlighting, for editing NML files. Microsoft Visual Studio Code is a free editor that is available for Windows, macOS, and Linux.
You can compile NML files to GRF resource files with NML, which is a free Python-based compiler. NML is available for Windows as a standalone program. It is also available for other operating systems but requires you to install the NML compiler and other required libraries separately.