There's an existing WordPress install having a thriving users list.

You will find several new Web programs being built-in Rails to pay for newer and more effective functionality.

Any suggestions or tips about integrating sign-on between Wordpress along with a Rails application?

Anybody done anything such as this before?

Something similar to this? Rails Integration API, although it may be the alternative of what you would like (i.e. it can make Wordpress speak with existing rails authentication system, not the other way round).

Otherwise, you can easily write some code to have interaction directly using the Wordpress database. This appears to become the best choice at this time.

When the two programs are running on a single subdomain, you'd have the ability to consider the cookie that is set when you are signed into wordpress. When the cookie can there be, you are able to grab the member info in the Wordpress database, while using session id within the cookie.