Haneda airport is one of the two major airports of Tokyo. The main transit to the airport is Keihin Kyuko Line (Keikyu) that departs from Shinagawa station in Tokyo.
I thought the train line was built for the airport, but, it was not. In fact, it was built for transporting visitors to a shrine at the location where Haneda airport is. The Anamori Inari Jinja was established around 1815 or so.
The Anamori train line opened in 1902 (Meiji 35), connecting Kamata and Anamori. The construction of Haneda Airport began in January 1930, and it oficially opened on August 25, 1931 (Showa 6). This means the train line existed well before the airport was established. In 1913, the train line was extended to Shin-Anamori, located directly in front of the shrine (see the image below) . During this time, commercial air travel was not yet common, and the area was wewell developed as a popular tourist destination for day trips, promoted by the train company. Attractions in the area included a horse racing track and a beach, making it a favored destination until the World War II started.
After the end of WW II in Septermber 1945, the occuping US army took over the Tokyo Airfield and surrounding neighborhood, and named as Haneda army air base. The US army demolished the area including the shrine and built an air base. (It looks like the island part beyond the bridge)
The train line was also shortened to Inari-bashi station.
The airport was returned to Japan in 1952.
The commercial airport terminal building was opened in 1955. Haneda Kukou (airport) station(羽田空港駅) was opened in 1956. However, the location was right before the river, and the passengers needed to walk or use bus to the airport terminal, but the access was not very good. At this time, majority of the passengers were taking Tokyo monorail.
Anamori line was renamed to airport line (空港線) in 1963.
After the expansion of Haneda airport, the train line was finally extended to below the new terminal. With the much improved access, this line becomes the most popular way to get to the airport from downtown Tokyo.
Reference
https://trafficnews.jp/post/119854 https://ja.wikipedia.org/wiki/%E4%BA%AC%E6%80%A5%E7%A9%BA%E6%B8%AF%E7%B7%9A#%E6%AD%B4%E5%8F%B2