I think that the Cart Titan should have just been a regular Titan.  The Cart Titan has absolutely nothing special about it (other than hardening, which most Intellegent Titans have)  There have been several Quadrupedal Titans seen in Attack on Titan, one of which being MUCH bigger and faster than the Cart Titan (The one that chased Sasha in the Female Titan arc)  If the Quadrupedal was just a regular Titan with intellegence, then there could've been an interesting story for another unique Titan.  Who knows it could've even had it's own arc just like the Female Titan did.  

TL;DR  The Cart Titan is a waste of an Intellegent Titan and easily could've easily been a Pure Titan