Seoul Line 5 forks at Gangdong, and we picked the wrong branch first
Two branches leave the same station. We decided the longer one was the main line. The station numbers say otherwise.
October 1, 2026 · 2 min read
Line 5 runs across Seoul from Banghwa in the west, through the center, and out to the east. At Gangdong it splits into two. One branch carries on east and then south-east to Sangil-dong and beyond, and the other turns south to Macheon. A map of a forked line asks you to decide which fork is the line and which is the spur.
The mistake
When we first added Line 5 we chose by length. The Macheon side is the longer one, so we made it the main line and drew the Sangil-dong side as a decorative spur. It seemed reasonable. Then someone asked a simple question: wasn't the Hanam side the main line, and Geori the branch? We looked it up and found we had it backwards.
What the numbers say
Seoul stations have codes, and the code tells you the structure. The official main line is numbered in one run: Banghwa is 510, Gangdong is 548, Sangil-dong is 553, and the Hanam extension, which opened in 2021 with Gangil, Misa, Hanam Pungsan, Hanam City Hall and Hanam Geomdansan, carries it on to 558. The Macheon side uses a different prefix, P549 to P555, and Korean Wikipedia gives it an article of its own, as a branch. So the long way round is the branch, and the line that goes to Hanam is the main line.
What it changed
We swapped them. In the game, the main line now runs from Banghwa to Hanam Geomdansan, and that is the route you type. The Macheon branch is still drawn on the map as a dashed extension, but it is not part of the typing route. It changed other numbers too. While checking the codes we found that Yeongdeungpo Market was missing from our data, which had pushed Singil's code off by one, and that Gangdong-gu Office had been put on Line 5 when it is actually a Line 8 station.
A rule we kept
The lesson was a rule: when a line forks, do not pick the main line by counting stations. Look at how the operator numbers the stations, and at whether the encyclopedia treats a stretch as its own branch. Both are cheap to check and both were right. Our Line 5 page now follows them.
Sources
More stories
HistorySix first subways, and the cities that built themFrom a gas-lit steam line in 1863 to Seoul's Line 1 in 1974, how six cities opened their first underground railway.
HistoryWhy the New York subway has numbers, letters and three historiesThe map looks like one system because it has been one system since 1940. Before that it was three companies that did not much want to cooperate.
HistoryThe Tube map is a diagram, and that is why it worksHarry Beck drew it in 1931 after losing his job. It throws away distance on purpose, and that is the whole trick.