What you could do is make a 3D undercarriage, but not TOO 3D detailed, to avoid framerate issues. I'd say only make a few of the key undercarriage elements 3D (Like the wheels, a few of the boxes and stuff under there, etc) and make all the rest (Barely noticable parts, everything else) just basic texture panels. And since the front area of the undercarriage (Front coupler, etc) are the undercarriage parts that are most often seen from outside, that area of the undercarriage should be fairly detailed (Again, FAIRLY detailed, with 3D parts, but do not overdo it)