Located in ./GitHub/RailCOMPLETE-DNA-[XX-YY]/[XX-YY]/DNA/_SRC.
_SRC contains the source files for the main DNA xml file. They can be collated by batch files using Xppq. The files are organized by subject matter. For help editing the content of the files, see the DNA Agent User Guide. This is a text file bundled with the RailCOMPLETE software, typically located at %appdata%/Autodesk/ApplicationPlugings/RC.bundle/Agent/ if RailCOMPLETE is installed on your system. It can also be found on the railcomplete.com download page.
See here for a general guide on XML.
For details on the RC data model, see
The source files for the DNA xml-file are xml stubs that are collated together with XPPQ. The file [Adm code]-Rootfile.xml is the xml root. It must contain references to all other files using tags like <xpp:expand href="[File name]"/>. The content of these referenced files must be contained in <xpp:bloc> xml elements.
XPPQ is also used for reusing snippets of xml around the code. This is done by using the element <xpp:define name="[Macro name]">, which should contain a <xpp:bloc> element. The snippet contained in the <xpp:bloc> can then be pasted anywhere in the code by calling <xpp:expand select="[Macro name]"/>.
The _SRC XML files frequently make use of Lua code. It is typically found inside LuaFunction elements. The primary use of these Lua functions is to set formulas on object properties.
See here for a general guide on Lua.
RailCOMPLETE Lua also has access to the RailCOMPLETE Lua API. See the F1 documentation inside RailCOMPLETE for details on this.
[Explanation]
See here for a general guide on DotLiquid.