Here's an excerpt from the developer's docs (9.01 Create an Engine.pdf).
It gets interesting at item 10, and especially the final note.
5.1 Create the Engine Blueprint
1. Open up the Asset Editor from the shortcut you have create
2. In the Blueprint Editor window, find your SimpleEngine in the folder structure
3. Right click the Engine folder name and select New Blueprint from the list
4. Select Engine Blueprint from the menu that appears and then click OK
5. Right click on the Engine blueprint000 file in the folder tree
6. Rename the file SimpleEngine
7. In the top Name field type SimpleEngine
8. In the Browse Information section type SimpleEngine in the language fields
9. In the Mass field, type 50. This is the weight in Tons
10. For the front pivot X, input 9.2 and the back -9.2
11. For Collision Centre Y, we need the centre of the main body sat on the wheels. This is
calculated by taking the full height of the vehicle (3.75m) subtracting the value below the main
body (1.25m), dividing the remainder my 2 (2.5m) and adding back the height below the main
body = 2.5m
12. For the Collision width, type 2.5
13. For the Collision height, type 2.5
14. For the Collision length, type 18m
Note - this is less than the pivot points at either end. This is so the coupling points are not inside the hit
box.
EDIT: Also this from 2.07 Wagon Blueprints.pdf:
3.1.18 Collision Length
The Collision Length should be slightly smaller than the real dimensions of the
vehicle, so that tight corners don’t cause Collision Boxes to overlap. This value
should be over the buffer beam, but not the buffers. The front/rear pivot points
take the buffers into account.
If your collision length includes the buffers the vehicle will struggle to couple up
as it will bounce off other vehicles.
