Diesel locomotives should not be able to move immediately after being fueled. They should have a slow startup and warmup period (though not nearly as slow as firing a steam locomotive, and depending on the engine size determined in the locomotive's .json) before they can move. If left idling in neutral, they will still use a small amount of fuel. But if they are shut off completely, the player must go though the startup and warmup process again, depending on the engine temperature.