Program for creating language syntax/grammar "railroad" diagrams?

As you can see by my edits to your question, the kind of syntax diagram you're referring to is also known as a railroad diagram. Wikipedia mentions a few tools under the "External Links" section, including:

• EBNF Visualizer

• Ebnf2ps: Peter's Syntax Diagram Drawing Tool

• EBNF Parser & Syntax Diagram Renderer

• SQLite bubble-generator.tcl

See also these related Stack Exchange questions:

• Super User: Software to draw/edit the Syntax of SQL statements

• Stack Overflow: Tool for generating railroad diagram used on json.org

• Stack Overflow: What is a good tool for creating railroad diagrams?

• Stack Overflow: What's the name for this type of flowchart?

• Stack Overflow: Software for creating “Language Diagrams” structures

• Stack Overflow: BNF notation to SVG