Look at line 17: https://github.com/visionmedia/express/blob/master/lib/request.js#L17
It inherits IncomingMessage which, according to the sources, is a base class for ServerRequest. Since I couldn't find any IncomingMessage definition, I assumed that ServerRequest is the closest one.

Also looking at the express.Response object, it inherits http.ServerResponse so it's only logical to make the same assumption for the express.Request object.