No, this post is not about transportation - though that might have been a good topic as well.
This post is about the structure of words.
In the mallu grammer study, the great Koyi Thampuran prescribed "പദമില്ലൊറ്റമാത്രയില്്", that is to say, there are no words in one മാത്ര. A മാത്ര is the basic length of sound - equivalent to that of a short vowel.
So when we adopted words like 'bus' and 'car' into mallu, we added our friend സംവൃതോകാരം to the end. This adds another മാത്റ so these can be proper words. Thus, "bus" became "bussu". Similarly, "switch" became "switchu" etc.
Now, if you were very discerning, you would have noticed that "car" has two മാത്റs , so technically it can be a stand alone word. So why should it need the additional 'u' at the end?
I actually don't know the answer to that. Looking at similar two മാത്റ words, we can see that many of them acquire the 'u' at the end. E.g. തീ -> തീയ്, പൂ -> പൂവ്, കാ -> കായ്, ചോര്് -> ചോറ്.
So my explanation is that those two maathra words were considered as roots. Note that if the two maathras are because of different letters then it is "okay", e.g. തല, പന