The main screen contains four basic areas, as shown in the following screenshot:

The preview area shows the preview of the loaded vehicles in the selected direction.
The vehicles area allows the user loading/selecting vehicles and interacting with them by clicking each of the six available slots with the right mouse button.
The view direction area contains eight buttons that allow the user changing the current direction loaded into the preview area.
The controls area allows the user to move the selected vehicle (align it) in the preview, as well forcing a preview refresh by using the refresh button.
Each slot in the vehicles area allows the user to left-click select the slot as the active one. Also right-clicking the slot will show the contextual menu, allowing to load a vehicle, remove it, save it or change its length.
If a same vehicle is loaded in different slots the modified settings for any of its slots will affect to all other slots with the same vehicle.
When a vehicle is saved it will be automatically moved inside the saved image to fit the current alignment shown in the preview area.
There are eight buttons that allows the user to change the active preview to any of the eight directions shown by the buttons. By clicking a button will automatically refresh the preview to the selected direction.
The controls area contains a refresh button that will force the preview area to reload all current settings.
There are also four arrow buttons that allow the user to move the currently selected vehicle (in the active slot) to move in the preview area. The preview area will be automatically refreshed to show the updated vehicle position.