A step-by-step walkthrough of building a remote coding agent platform on Railway, a cloud deployment platform. The project creates a control plane that dynamically provisions and decommissions sandbox environments containing AI coding agents (OpenCode, Claude Code, Codex CLI) and a VS Code server interface. The architecture includes a TypeScript/Express API backed by PostgreSQL (via Drizzle ORM), a React/Vite frontend, and proxy routing so users can access sandboxes through a single authenticated endpoint. Key steps covered include containerizing OpenCode and code-server, using the Railway GraphQL API to create/delete services programmatically, setting up the Railway MCP server and skills for agent-assisted deployment, implementing JWT-based auth with WebSocket proxying, and adding health-check polling for sandbox status indicators. The entire build process itself uses AI coding agents (primarily OpenCode) to generate and debug the code.