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Trip planners like Rejseplanen, Citymapper and Google Maps tell you how to get somewhere. None of them shows the whole city moving at once. Mylder is for that view first, with useful info (departures, delays, disruptions) one tap away.
Honest data states
Live transit data is patchy: some vehicles report their position, some are only estimated, some have no realtime at all, and answers stop coming. Mylder never hides which is which. Every vehicle and departure carries one of these states:
State
Meaning
Example label
Live
Reported by the vehicle, within about two polls
"Reported 20 s ago"
Ageing
Reported, but older than that and under 5 min
"Last seen 3 min ago"
Estimated
Computed by Rejseplanen, or worked out by Mylder from the timetable and realtime
"Estimated from schedule"
Timetable only
No realtime for this trip today, like every Metro train
"Timetable only"
No signal
Had realtime, then missing for 5 min
"No signal since 10:42"
On the map, reported positions are solid dots and estimated ones are hollow. A position Rejseplanen computes is never shown as Live. Thresholds are first guesses until real coverage is measured. The full rules are in docs/data-states.md, and the words in GLOSSARY.md.
How it works (planned)
Rejseplanen API ──▶ one Next.js app on Vercel ──CDN──▶ browsers
(positions, one call a minute while poll one snapshot,
departures, someone watches; joins it animate vehicles
disruptions) to the Labs GTFS timetable along their paths
The free Rejseplanen key allows about one call a minute, so browsers never call the API. The app's server makes that call only while someone is watching, joins the answer to a timetable prepared from Rejseplan Labs GTFS, decides each trip's data state, and serves one snapshot of the whole city through the CDN. Every browser polls that snapshot and moves each vehicle along its path until the next one. The decisions and their reasons are in docs/adr/; the plumbing in docs/live-data.md, docs/timetable.md and docs/screens.md.
Roadmap
First public version
App shell: Next.js, React 19, TypeScript, Vitest, Playwright, CI
Live map with every vehicle moving along its route (MapLibre GL + deck.gl)
City pulse: vehicles moving now, share reporting per mode, and average delay if the data can back it
Stop view: next departures, each with its data state
Vehicle list: every vehicle on the map in a table, sortable by line, destination and data state
Disruptions on one screen
English first, Danish second, mobile first
Later: harbour ships from AIS, car traffic flow, replay of a past rush hour, a full-screen kiosk mode, more cities.
Agent skills in .claude/skills/: grill-me, grilling, wayfinder (with domain-modeling, prototype and research), to-spec, to-tickets, implement-spec (with tdd, codebase-design and code-review), improve-codebase-architecture and software-factory from mattpocock/skills (MIT), and vercel-react-best-practices from vercel-labs/agent-skills (MIT)
Planned: Vercel for deploys, Effect for the server's tracker, MapLibre GL and deck.gl for the map, and Perron for every component and token
Getting started
Requires Node 22 or newer.
npm install
npm run dev
The interface lives at /en and /da; / sends a visit to the language its browser asks for, English when it asks for neither.
Before the first e2e run, install a browser with npx playwright install chromium.
Script
What it does
npm run dev
Start the Next.js dev server
npm run build
Build for production into .next/
npm start
Serve the production build
npm run check
Type-check, lint, format check and unit tests
npm run typecheck
Type-check with tsc
npm run test
Run unit tests once
npm run test:watch
Run unit tests in watch mode
npm run test:coverage
Run unit tests with coverage
npm run test:e2e
Build, serve and run Playwright tests
npm run demo
Build the GitHub Pages demo into out/
npm run demo:serve
Serve the built demo as GitHub Pages does
npm run test:e2e:demo
Run the demo's Playwright tests on its build
npm run recordings
Freeze the test recordings from the simulation
npm run lint
Lint with oxlint
npm run format
Format everything with Prettier
Folders
Each folder's name says where its code may run. Three are guarded, so a wrong import fails the build or the lint:
Folder
Runs on
What it holds
Guard
app/
Server
Next.js routes and route handlers, kept thin
src/domain/
Both
Pure rules: data states, positions, delays
npm run lint fails on any import from another folder
src/tracker/
Server only
The Effect tracker behind the snapshot
server-only: next build fails in a client module
src/timetable/
Both
The prepared timetable's format and readers
src/map/
Browser only
MapLibre, the deck.gl layer and the animation loop
client-only: next build fails in a server module
src/screens/
Browser
Client components for the screens, built from Perron
src/i18n/
Both, loader on the server
English and Danish dictionaries and the language type
src/demo/
Web Worker, demo only
The simulated tracker of the GitHub Pages demo
Tests sit beside the code they test; Playwright journeys are in e2e/, and the timetable job in scripts/timetable/.
Data
Transit data from Rejseplanen Labs, used under its non-commercial guidelines. API keys stay on the server and are never committed.
About
Built by Marcin Sieradzki, a frontend and AI engineer in Copenhagen.
