Inspiration

As a kid, I spent countless hours on subway builders like JP Wright's Brand New Subway.

Architecture

Database

Transit Planner uses PostgreSQL for its PostGIS extension.

Frontend

Transit Planner uses Next.js, TypeScript, and is styled with Tailwind.

Backend server: LLM Setup

Transit Planner uses Anthropic and Gemini APIs, allowing both as options to users. Dialog is streamed via SSE (FastAPI StreamingResponse), and the client renders tokens as they arrive.

Structured routes: tool schema

Free-form text is great for reasoning, but the map needs structured data like coordinates and stop order. After a council member's reasoning, it must emit a propose_route tool call. The vendor's tool-use layer validates it against inputSchema, as shown below.

Constraints like minItems/maxItems on stops and required fields keep generations in a band the renderer expects (e.g. 6–20 stops, hex colour, [lng, lat] pairs per stop).

const PROPOSE_ROUTE_TOOL: ToolDefinition = {
  name: "propose_route",
  description: "Submit the proposed subway route after your written analysis.",
  inputSchema: {
    type: "object",
    properties: {
      name:  { type: "string" },
      type:  { type: "string", enum: ["subway"] },
      color: { type: "string", description: "Hex colour, e.g. #2563eb" },
      stops: {
        type: "array",
        minItems: 6,
        maxItems: 20,
        items: {
          type: "object",
          properties: {
            name:   { type: "string" },
            coords: { type: "array", items: { type: "number" }, minItems: 2 },
          },
          required: ["name", "coords"],
        },
      },
    },
    required: ["name", "type", "color", "stops"],
  },
};

Launch

The initial launch received 1.4K likes and positive feedback on X.

More features

We're excited to develop this further. Here are some features I've added since:

Surface Route Accuracy

Initially, bus and streetcar routes floated from one point to another. I added automatic snapping to roads as well as portals to indicate underground sections that don't have to follow road medians.

This project has introduced me to much of the small but active community of transit data and modelling professionals, from whom I have a lot to learn from.

I hope to make this project either useful professionally or as an educational tool or game for transit enthusiasts. If you have any feedback or suggestions, or would like to collaborate, please reach out!