How I Built a Browser-Based GTFS Viewer That Runs Entirely Without a Server

A few months ago I found myself repeatedly opening the same clunky workflow: download a GTFS ZIP, unzip it, open stops.txt in a spreadsheet, try to make sense of route shapes from raw lat/lon coordinates. There had to be a better way. So I built TransitLens — a browser-based GTFS viewer and analysis tool that runs entirely in the browser. No server, no installation, no account required. Drop in a ZIP or paste a feed URL and you're looking at an interactive map of every route and stop in seconds. Here's what I learned along the way. Why browser-only? The obvious approach would be a backend: upload the file, parse it server-side, return JSON. But that creates real friction for the audience I was building for — transit agencies and developers who are often cautious about sending feed data to a third-party server. Some feeds contain schedule data that agencies consider sensitive before publication. Running everything client-side solves this cleanly. The file never leaves the browser. There