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