The red G indicates that your speed is too high with respect to the brake curve. You are not decelerating enough to meet the maximum speed at the next signal and therefore the emergency brakes are applied.
It also shows when you are exceeding max speed of PZB mode. Those speeds for modes are: O-165 km/h (Passenger) M-125 km/h (Freight) (Actually, perfect for br 143) U-105 km/h (Heavy Freight)