Complex sentence: Line 2 is the only line in Shenzhen that all stations of it are underground and are in Shenzhen Special Economic Zone, however it is the least used line in the system with only 247,500 passengers a day.
Split into simpler sentences: Line 2 is the only line in Shenzhen solely with underground stations that are inclusively in Shenzhen Special Economic Zone. However, it is the least used line in the system with only 247,500 passengers a day.