The only freeware rack & pinion loco I am aware of is kuid:44640:2 - a 3ft6in Fell H class Incline Locomotive by Greg Baker. It used to be available on his site, but a quick check seems to indicate that the site has been reconfigured for TRS2006 only (there are no current downloads available) and the rather nice downloads previously available have been withdrawn.
I have the item and the licence does not preclude me from distributing it to you via email if you so desire - dunno how big the CDP would be (or I could just Zip the directory), 2 1/2 megabytes or so I should imagine.