Critical infrastructures for, e.g. the transport of goods and people or the supply of energy and water, are the lifelines of our globalized society. Impairments can result in enduring supply bottlenecks, significant disruptions to public safety or other dramatic consequences. Currently used monitoring processes for these infrastructures are costly, difficult to implement in isolated areas, nonuniform due to heterogenous technical solutions and poorly scalable or automatable. The project Instantaneous Infrastructure Monitoring by Earth Observation (IIMEO) contributes to an end-to-end solution for the operational real-time, high-resolution monitoring of critical infrastructure by means of an innovative observation payload for a future LEO constellation. Our system concept focusses on data availability within less than 1 hour from user request to information delivery by AI-based processing approaches implemented on space-qualified on-board hardware. To provide a persistent weather-independent monitoring service with an improved spatial resolution of up to 50 cm, we propose a novel sensor configuration consisting of a 35-GHz-SAR sensor in combination with optical cameras. Based on existing technology from scientific institutions and European space industry, hard- and software will be further developed up to TRL 6. Development will be carried out in close cooperation with a railway company as a pilot user to define use cases for commercial applications based on the requirements of industry and public services. The end-to-end prototype service including on-board processing will be demonstrated within a final flight campaign. A roadmap will describe the further exploitation of the project results and outline further applications of the technology to other infrastructure systems. A follow-up demonstrator mission in 2026/27 is envisaged to showcase the monitoring of railways from space on a global scale on TRL 7.