Cart Mounts
There are three types of mounts, only equippable to the Cart Titan. They can be equipped in titan form if the Cart holds a mount and presses the spectatorOutlines key (it's unbound by default; check controls). Other players can equip mounts on the Cart Titan, but only the Cart can unqeuip them by sneaking and pressing the key again.
-
Saddle Mount

The saddle mount lets up to two players ride on the Cart's back.
-
Barrel Mount

The barrel mount gives the Cart Titan two barrels worth of extra storage, accessible only to the Cart with the spectatorOutlines key.
-
Rifle Mount

The rifle mount attaches a heavy anti-titan rifle on the Cart's back. The rifle has a 1/4 chance to blow itself up on use (configurable), which probably won't kill the Cart, but it will completely destroy the mount.
Commands
The cart mounts can be given with the usual /give command; i.e. /give @s titan_shifter_megapack:titan_saddle_mount.