Now that more duplo trains and cars are modeled, we should think about the primitives for the hitches aka tow pins, tow loops, and tow clips. Duplo hitches come in many types and variants. However, when minor differences are ignored, it boils down to a handful tow pins and tow hooks.
Long unreinforced tow pin
Suggested archetype: 4883a in LDraw
The classical tow pin has a long neck and no topside reinforcement.
4559, 4580 (Corrected), 4883a, 4883b, 53053, 31300 (Corrected), 59273a (Rebrickable)
Long fully-reinforced tow pin
Suggested archetype: 22307 in Rebrickable
The classical tow pin with long neck and full topside vertical reinforcement.
31507 (Corrected), 22307 (Rebrickable), 59273b (https://www.ebay.com/itm/365018516343)
Short unreinforced tow pin
Suggested archetype: 2312 in LDraw
This tow pin has short neck, no topside reinforcement.
2312, 31300a, 4580b, 6505
Clasped tow pin
Suggested archetype: 52071 in Rebrickable
This tow pin has short neck and horizontal triangle covering the topside
1326, 2034, 18520, 52071, 54007
Short fully-reinforced tow pin
Suggested archetype: 11248 in LDraw
This tow pin has short neck and full topside vertical reinforcement.
11248, 31300b, 15313, 15314, 31174, 95462, 98214, 58234, 53899, 4354
Short half-reinforced tow pin
Suggested archetype: 28759
This tow pin features a curved topside reinforcement.
28759, 28743, 2092
Tow loop
Suggested archetype: 4883a in LDraw
Features a closed tow loop/hook
4883a
Rectangular tow clip
Suggested archetype: 53053
Features an open tow clip/hook on a triangular rectangular support.
2320, 6505, 53053
Rounded tow clip
Suggested archetype: 2312&11248 in LDraw
Features an open conically-tapered tow clip/hook on a rounded support.
2312, 11248, 31174, 52071
Untapered tow clip
47450
Trailer tow clip with wheel, horizontal
18520, (87657), (87658), (89198)
Trailer tow clip with wheel, angled
14923
This list is not exhaustive. Some additional cases were omitted.
4883b reinforced clip/hook (https://www.ebay.com/itm/186383419871)
4820 reinforced trailer clip/hook
All movable clips/hooks
Long unreinforced tow pin
Suggested archetype: 4883a in LDraw
The classical tow pin has a long neck and no topside reinforcement.
4559, 4580 (Corrected), 4883a, 4883b, 53053, 31300 (Corrected), 59273a (Rebrickable)
Long fully-reinforced tow pin
Suggested archetype: 22307 in Rebrickable
The classical tow pin with long neck and full topside vertical reinforcement.
31507 (Corrected), 22307 (Rebrickable), 59273b (https://www.ebay.com/itm/365018516343)
Short unreinforced tow pin
Suggested archetype: 2312 in LDraw
This tow pin has short neck, no topside reinforcement.
2312, 31300a, 4580b, 6505
Clasped tow pin
Suggested archetype: 52071 in Rebrickable
This tow pin has short neck and horizontal triangle covering the topside
1326, 2034, 18520, 52071, 54007
Short fully-reinforced tow pin
Suggested archetype: 11248 in LDraw
This tow pin has short neck and full topside vertical reinforcement.
11248, 31300b, 15313, 15314, 31174, 95462, 98214, 58234, 53899, 4354
Short half-reinforced tow pin
Suggested archetype: 28759
This tow pin features a curved topside reinforcement.
28759, 28743, 2092
Tow loop
Suggested archetype: 4883a in LDraw
Features a closed tow loop/hook
4883a
Rectangular tow clip
Suggested archetype: 53053
Features an open tow clip/hook on a triangular rectangular support.
2320, 6505, 53053
Rounded tow clip
Suggested archetype: 2312&11248 in LDraw
Features an open conically-tapered tow clip/hook on a rounded support.
2312, 11248, 31174, 52071
Untapered tow clip
47450
Trailer tow clip with wheel, horizontal
18520, (87657), (87658), (89198)
Trailer tow clip with wheel, angled
14923
This list is not exhaustive. Some additional cases were omitted.
4883b reinforced clip/hook (https://www.ebay.com/itm/186383419871)
4820 reinforced trailer clip/hook
All movable clips/hooks