The line was initially announced in July 1998 as the Incheon International Airport Railroad (인천국제공항철도), abbreviated IREX [Incheon Airport Railroad Express] (which can be seen on the railings on overhead crossings by the line). The project was launched as South Korea's first build-operate-transfer (BOT) franchise. The concession was won by a consortium of 11 Korean companies, which incorporated as the Incheon International Airport Railroad Company (Iiarco) in March 2001. The original main shareholders were Hyundai (27%), POSCO (11.9%), Daelim (10%), Dongbu (10%) and the Korean National Railroad (9.9%).
After significant delays, construction of the line started in 2001, after the opening of Incheon Airport. About 60% of the line is underground, and reaches Yeongjong Island, which hosts Incheon International Airport, on the lower deck of Yeongjong Bridge, a combined road-rail bridge. Iiarco contracted project management, the supply of equipment and trains, altogether worth about €400 million, to Incheon Korean French Consortium (IKFC), a consortium including French company Alstom, its South Korean subsidiary Eukorail, and the South Korean rolling stock manufacturing company Rotem, which also supplied the technology for the KTX high-speed rail system.
The line and the operating company was renamed AREX in June 2006. The first 37.6 km (23.4 mi) segment from Incheon International Airport Station to Gimpo International Airport Station opened on March 23, 2007.
In March 2009, Korail acquired an 88.8% share of the company. Upon the acquisition, the operating company changed its name to Korail Airport Railroad as of November 30, 2009.
Phase 2 of the line is from Gimpo International Airport Station to Seoul Station, which increased the length to 58 km (36 mi). Including a depot section, the full length of the line is 61.7 km (38.3 mi). At Gimpo Airport the line is underground and heads north-east and rises to the surface to cross the Han River on the Magok Railway Bridge. The line descends into a tunnel and curves to the east. The line was built in a cut and cover tunnel running parallel to Susaek Station and Gajwa Station before connecting with Hongik University and Gongdeok-dong before terminating deep under ground on the west side of Seoul Station. The second phase opened for regular service on December 29, 2010, with the exception of Gongdeok Station, which was planned to open around early 2011, was pushed back to December 2011 and then opened November 29, 2011.
The city of Incheon requested that AREX add three new stations, tentatively named Cheongna International City, Yeongjong and Yongyu. The stations were originally planned to open with the second phase of the line, but were delayed due to delays in the real estate development projects the stations are to serve. Cheongna International City Station, located west of Geomam was opened on June 21, 2014. Yeongjong station, located near the eastern part of the airport island, opened on March 26, 2016. The station was constructed as a station shell with two side platforms only, but underwent a conversion to fit in two outer tracks on each platform to allow for bypass moves.
Earlier plans to link Yongsan Station have been dropped. However, KTX service began to run to Incheon International Airport on June 30, 2014. The trains stops 2 stations on the line: Geomam & Incheon International Airport. On September 1, 2010, the South Korean government announced a strategic plan to reduce travel times from Seoul to 95% of the country to under 2 hours by 2020. As part of the plan, the AREX line is to be further upgraded for 230 km/h (143 mph).
AREX connects to Incheon Subway Line 2 from July 2016 via Geomam Station.
A second transfer station with Line 9 has been built but yet to open due to the lack of development in that area. It is unclear when Magongnaru Station will be open to the public.
Incheon International Airport has a plan to build a second passenger terminal located in north area of the airport by 2020. The AREX line would be extended to the second passenger terminal.
AREX operates two types of trains, one for commuter and one for express services, both supplied by Hyundai Rotem. Three 1000 series express trains and nine 2000 series commuter trains were delivered for phase 1 of the project, and original plans foresaw a doubling of their numbers for the second phase.
Both trains are six-car electric multiple units, which can be expanded to eight cars in the future. End cars and the fourth car are trailers, the second, third and fifth car are powered. The trains have a design speed of 120 km/h (75 mph), but are operated at a maximum 110 km/h (68 mph) on surface tracks and a maximum 100 km/h (62 mph) in tunnels. Each car has a length of 19.95 m (65.5 ft) and a width of 3.12 m (10.2 ft), with a carbody made of aluminum. The commuter version has four pairs of sliding doors on each side of each car, the express version has two pairs of sliding doors on each side of each car.
Express trains have 272 comfortable seats with armrests in 2+2 configuration, overhead luggage shelves and additional luggage racks. Passenger compartments are separated from door areas and from transitions between cars by transparent sliding doors. Commuter trains are similar to subway trains with seats along the walls and do not have overhead shelves, offering seating for 282 passengers and standing room for 630 passengers. As these are open gangway trains, there are no separating doors, and transitions between cars are open. Both train types offer disabled seats and are equipped with LCD screens for passenger information, including flight arrivals and departures.
Both express and commuter services are operated. With the opening of the first phase, journeys from Incheon International Airport to Gimpo International Airport took 28 minutes on express services and 33 minutes on commuter services. Following the opening of the second phase, Seoul–Incheon International Airport travel time on half-hourly express services is 43 minutes; while commuter services, which run at a frequency of up to one train every 6 minutes, take 53 minutes. This compares with an average travel time of 70 minutes on buses and 60 minutes in taxis or private cars. Airline passengers can check in at a western annex of Seoul Station.
The AREX line is shown in blue on Seoul subway maps, but was originally not considered part of the subway network. With the majority buyout by a company that operates many subway lines (Korail), however, this has changed. There are now free transfers for Line 5, Line 9, and Incheon Subway Line 1.
From 2020, the AREX line is proposed to be used by Seoul Subway Line 9 trains via a direct connection at Gimpo Airport Station, providing a through service from Incheon International Airport to the Gangnam area, with a travel time of about one hour.
The opening of the second phase in 2010 allowed passengers to connect directly from KTX to AREX services at Seoul Station, rather than requiring a subway connection at Gimpo. In November 2013, the AREX-Gyeongui Line direct connection was completed, and on June 30, 2014, KTX trains started full service on the AREX line to Incheon. Expected travel time at construction was 2 hours 41 minutes to Busan and 3 hours to Gwangju; as of 2015, actual travel times were roughly 3 hours to Gwangju, but slightly over 3.5 to Busan.
After the opening of the first phase, in 2007, a ticket for a ride from Gimpo International Airport to Incheon International Airport cost ₩3,100, both on commuter and express services. This fare increased in steps to ₩3,500 by the end of 2010, then was reduced to ₩3,300 for commuter trains when the second phase opened. The fare for express services, which travel the full Seoul–Incheon International Airport distance non-stop from the opening of the second phase in December 2010, was set at ₩13,300; while the fare for commuter services on the full distance was set at ₩3,700. This competes against a price of around ₩10,000 on buses, ₩67,500 on taxis, and about ₩15,500 with private car.
After the opening of the first phase, ridership has been far below expectations. Actual ridership in 2008 was around 16,000 passengers/day, or only around 7% of the forecast 230,000, necessitating subsidies of ₩166 billion. Hyundai had originally forecast ridership of up to 490,000/day when the link to Seoul Station is completed. However, ridership rose significantly after the start of service on the second phase of the line to Seoul Station. As early as the first week, average daily ridership more than doubled, from 27,210 to 58,000.
As of September 2013, the regular train service has approximately 150,000 passengers per day, while the express sees only around 2,000 passengers.