You have to wait because a rail-crossing barrier is lowered. How can you now help protect the environment at this point?

Your answer is correct. When waiting at a closed railway crossing, you help protect the environment by using the automatic start-stop system. This system turns off the engine automatically while you wait and restarts it when you're ready to move again. Letting the engine run unnecessarily creates air pollution and wastes fuel. Shutting off the engine (manually or with the automatic system) reduces emissions and saves fuel, which is better for the environment. The other actions, such as putting the car in first gear or using the parking brake, do not reduce emissions while waiting. So, always switch off your engine during long stops whenever it is safe and practical.