There are non-trivial numbers of diesel-mechanical (IC to wheels via a gearbox) and diesel-hydraulic (IC to wheels via a fluid coupling) trains out there. The British class 172 Turbostar is an example of the former, most other class 1xx DMUs the latter.

Diesel electric transmission does happen on smaller multiple units sometimes, but really shines on larger and faster trains, or on electro-diesels (where you need the electric final drive anyway).