Let's Write a Train Tracking Algorithm

A presentation at iOSDC 2025 Japan exploring the algorithm behind Eki Live, an iOS app that automatically detects the railway and next station for a train you're riding in Tokyo and updates it in the Dynamic Island.

Transcript

1. Let's Write a Train Tracking Algorithm Chris Trott iOSDC 2025/09/20
2. Hi, I'm Chris ! From Chicago " Now Japan (~8 years)
4. Let's Write a Train Tracking Algorithm
5. What is a train tracking algorithm?
8. Talk Overview 1. Review Data Prerequisites 2. Write Algorithm
9. Data Prerequisites — Static railway data — Live GPS data from an iPhone on a train
10. Static railway data — Railways — Stations — Railway Directions — Railway Coordinates
11. Static railway data — Railways — Stations — Railway Directions — Railway Coordinates
13. GPS data — Local SQLite Database — sessions & locations tables
14. Location
15. Session
17. Write Algorithm 1. Determine Railway 2. Determine Direction 3. Determine Next Station
23. Example 1 — Railway: Toyoko Line — Direction: Inbound (to Shibuya) — Next station: Tsunashima
24. 1. Determine Railway 2. Determine Railway Direction 3. Determine Next/Current Station
26. Railway Algorithm V1 — Find closest Railway Coordinates to Location — Sort railways by nearest
27. Railway Distance from Location (m) Tokyu Toyoko 12.19 Tokyu Shin-Yokohama 177.19 Yokohama Green 1542.94 Tokyu Meguro 2266.07
29. However...
30. Example 2 — Railway: Toyoko Line — Direction: Outbound (to Yokohama) — Next station: Hiyoshi
31. Problem: Toyoko Line and Meguro Line run parallel
32. We need history
33. Railway Algorithm V2 — Convert distance to score — Add scores over time
34. Railway Algorithm V2
35. However...
36. Example 3 — Railway: Keihin-Tohoku Line — Direction: Northbound — Next station: Kamata
39. Equal?!
40. Railway Algorithm V3 — Add penalty for passed stations — Add penalty for stopping between stations
50. There are many more edge cases but...
51. Let's move on!
52. 1. Determine Railway 2. Determine Railway Direction 3. Determine Next/Current Station
53. Every Railway has 2 directions
56. Example — Railway: Toyoko Line — Direction: Inbound (to Shibuya) — Next station: Tsunashima
57. Direction Algorithm V1 — Mark timestamp for 2 stations — Compare order of 1st and 2nd station
60. However...
61. We must visit 2 stations in order to get a prediction...
62. Can we do better?
63. Use Location.course!
67. 359.6°
68. Direction Algorithm V2 (1) Fetch 2 closest stations to input location
69. Direction Algorithm V2 (2) Calculate vector between 2 closest stations for "ascending" direction
70. Dot Product
71. Direction Algorithm V2 (3) Calculate dot product between location course vector and closest stations vector
72. — Positive dot product == "ascending" — Negative dot product == "descending"
73. -0.95 → negative → "descending" → Inbound to Shibuya
74. Let's move on!
75. 1. Determine Railway 2. Determine Railway Direction 3. Determine Next/Current Station
77. — Next: Kawasaki
78. — Next: Kawasaki — Soon: Kawasaki
79. — Next: Kawasaki — Soon: Kawasaki — Now: Kawasaki
80. — Next: Kawasaki — Soon: Kawasaki — Now: Kawasaki — Next: Kamata
82. Focus Station Algorithm V1 — Calculate distance d from Location to closest station S — Calculate direction vector c from Location to S Case Result d < 200m "Now: S" d < 500m && c > 0 "Soon: S" c > 0 "Next: S" else "Next: S+1"
87. However...
88. GPS data is unreliable
89. Let's create a history for each station
90. struct StationDirectionalLocationHistory { let stationID: Station.ID let railDirection: RailDirection var visitingLocations: [Location] = [] var approachingLocations: [Location] = [] var firstDepartureLocation: Location? }
92. Focus Station Algorithm V2 — Step 1: assign locations to stations — Step 2: update station phase history — Step 3: select the focus station phase
93. Step 1: assign locations to stations — Assign visitingLocations or approachingLocations
94. Step 1: assign locations to stations — Assign firstDepartureLocation
95. Step 2: update station phase history visiting approach departure -> Phase isEmpty isEmpty !nil departure isEmpty !isEmpty nil approaching !isEmpty any nil visiting !isEmpty any !nil visited
96. approaching
97. visiting
98. visited
99. Step 3: determine focus phase — Find last station S in travel direction where phase != nil Latest Station Phase Focus Phase departure Next: S+1 approaching Soon: S visiting Now: S visited Next: S+1
103. Demo
105. github.com/twocentstudios/train-tracker-talk
106. Try Eki Live!
107. ✨ Hire Me ✨ — Available for full-time or contract work — iOS generalist (not just train apps) — twocentstudios.com/blog — @twocentstudios