TrainCarts/Attachments/Physical
The physical menu, available at the top (root) attachment, configures the length of the cart and where the wheels are positioned. This controls the spacing between carts and how the cart rotates in turns. The menu shows a bounding box preview of the current dimensions of the cart.
Cart Length
This controls the length of the cart. Multiple connected carts maintain distance between each other with a gap between them. This length controls the spacing this cart takes up in this linked chain.
Wheel Distance
This controls how far from the center of the cart the front and back wheels are positioned. Further-spaced wheels turn more smoothly. If your custom train includes actual wheels, it helps to position the wheels in roughly the same place for realistic tracking on the rails. The wheel positions are displayed using ender pearls.
Wheel Offset
If your wheels are not exactly centered, the offset allows for adjusting this. Usually this should be left 0.