Very similar is this: https://github.com/tabatkins/railroad-diagrams and its online dingus: https://tabatkins.github.io/railroad-diagrams/generator.html which appears to be a friendlier, if greener, implementation.

Very crisp output. I’ve tried to use Graphvis for things like this, but I can never figure out how to make boxes nicely aligned, or make arrows not squiggly.