Difference Engine
Ok, there's nothing here yet. I have made a prototype of the adding mechanism. It uses those curved railing sections. A full circle made with four of these has 20 slots that are wide enough to accept a tile piece. That's cool because not only is it nicely in base 10, but in Babbage's orignal engine the adding mechanism also used a 20 tooth cog.
In the mean time if you can't wait for me to build a LEGO difference engine then why not have a look at Andrew Carol's rather nice model. It's the first LEGO difference engine on the web and obviously plenty of kudos goes with it!