Ah this look like geometry problem
#Need Help with Minecarts
weary verge
dark gazelle
you didnt properly resize it
default texture size of minecart is 64x32, so it cant resize to 256x256
it can only resize to 256x128
and moreover, the texture is extremely small, only on the top left corner, hence why the texture looks bugged
harsh fern
You may need to change the texture uv in the model