The tunnel, which consists of a double-track tube, is part of the 61.1-kilometre (38.0 mi) Suseo–Pyeongtaek high-speed railway and GTX-A that connects Suseo station in the southeastern part of Seoul with the Gyeongbu high-speed railway.
The threading into the older high-speed line is in the south of the city of Pyeongtaek.
The average cruising speed is about 240 km/h (149 mph) due to the intermediate stop at Dongtan Station in the southern part of the tunnel.
The audit argued that several factors, including negligent geological surveying and unauthorized construction methods caused the trackbed to deform, rendering the track structure uneven which ultimately resulted in excess vibration in the trains, reducing the highest operational speed from 240 km/h to as low as 90 km/h in certain sections.
In the wake of these flaws the Korea Rail Network Authority were ordered to prepare a comprehensive reinforcement plan.