A through service is a concept of passenger transport that involves a vehicle travelling between lines, networks or operators on a regularly specified schedule, on which the passenger can remain on board without alighting. It may be in form of either the following:A service where the vehicle travels between different lines, or systems of infrastructure, for example, a through train service between the mainline and underground railways. A service where the vehicle changes its identity en-route without requiring passengers to alight, for example, a through tram service which runs as route 1 initially, then runs as route 2 for the latter half of the journey.

Seoul Subway Line 4 of the Seoul Metropolitan Subway is a long line crossing from the southwest to the northeast across the Seoul National Capital Area. The central section in Seoul City is operated by Seoul Metro with some trains offering through service to Korail's Ansan and Gwacheon Lines. The southern terminus (Oido) is in Jeongwang 4-dong, Siheung City, and the northern terminus (Jinjeop) is in Jinjeop-eup, Namyangju-si, Gyeonggi-do; the newly built northern section is correspondingly owned by a company owned by Namyangju-si, with operations contracted to Seoul Metro. In 2022, the Seoul Metro operated section had an annual ridership of 219,587,000 or about 601,608 passengers per day.