A reduction by 3–4 dB was achieved by retrofitting all trains with longer mud flaps at car ends until May 2006 to smooth the airflow at the articulated car joints. However, measurements in 2009 found significantly higher interior noise levels at some locations in two tunnels. Window thickness and sound insulation was improved in the KTX-II. The rails for high-speed trains like the KTX are welded together via a special techniques that make the rail a solid continuous rail; this method reduces the noise volume, which is produced by the wheels’ contact with the rail, but it is not fully eliminated.