In Steve's Carts, as many as three Engines can be attached to one cart. This can enable carts to use a Solar Engine during the daylight, and a Coal Engine at night, or when working underground. Engine Priority is the mechanism that controls which Engine is used at any given point. There are four priority levels: Disabled (red), Low (orange), Medium (yellow), and High (green). An Engine with a higher priority level will always be used before an Engine with a lower priority.
For instance, a Drilling cart that has a Compact Solar Engine (High Priority) and a Coal Engine (Medium Priority) attached, would operate for as long as it could on its charged Solar Energy while drilling underground. Once the Solar Energy is completely drained, it would switch to the Coal Engine, and operate on Coal power.