Taxi Simulator has at least four different unused taxis, which include the Taxi Chassis, the Hover Taxi, the Feature Testing Vehicle, and the Black Wall Driver. These taxis are only accessible in-game using exploits, any other unused Taxi when spawning will load forever.
The oldest of the unused Taxis is the Taxi Chassis, which is simply used as a tool to develop more Taxis by the game developers. It is minimalistic and only has the parts a Taxi needs to function, wheels, two seats, a block for an engine, a block for the trunk, and a license plate. It's color is Medium Stone Grey. This Taxi's model may be found at https://www.roblox.com/library/221170433/Taxi-chassis.
The Hover Taxi is a strange taxi with the wheel rims facing the ground. Likely early into development, it was scrapped for the Guestium Wheels, arguably a much better reward for beating Victor's Avenue than a Hover Taxi. The Taxi's only mode of transportation is, ironically, the Guestium Wheels. The Taxi has no programmed movement features besides the Guestium Wheels at all. Another Interesting fact about the Hover Taxi is that the headlights were not made properly and are off, while there is still an effect coming from them that lights up the area (see image). The Taxi can still be damaged as the Default Taxi can. This Taxi's model may be found at https://www.roblox.com/library/259748042/Hover-taxi.
Feature Testing Vehicle
The Feature Testing Vehicle looks and functions almost the exact same as the Default Taxi, with a few minor quirks. When you spawn this Taxi, it loads forever until you reset your character, yet you can still control it. The wheels spawn a considerable amount above the Taxi, meaning that the Taxi may not spawn correctly in areas with low ceilings, or in the Shop. It may equip as many upgrades as the player wants it to. It is important to note that this Taxi will almostly certainly completely change function in the future, as it is a testing vehicle. This Taxi's model may be found at https://www.roblox.com/library/286530532/Taxi-Simulator-Feature-Testing-Vehicle.
Elite Wall Driver
The Elite Wall Driver is a reskin of the Golden Wall Driver that makes it a dark black, the same black as the Master Champion Winner Taxi. It functions the exact same as the regular Golden Wall Driver, but its model name is "Golden Wall Driver With New Seat". This is the Taxi used in elite Taxi challenges Victor's Avenue, Perfectionist's Driving Surface, and Over Achiever's Lane. They look different because this Taxi uses a strong reflectance value and the challenges vary in lighting. This Taxi's model may be found at https://www.roblox.com/library/836570677/Golden-Wall-Driver-With-New-Seat.