The goal of the SEASON project is to design and validate a sustainable transport network infrastructure able to support beyond 5G and new emerging services. The SEASON infrastructure will rely on the joint usage of Multi-Band (MB) and Space Division Multiplexing (SDM), spanning the access, aggregation, and metro/long-haul segments, supporting the requirements for x-haul, further integrating the packet/optical and computing layers, and targeting cost-effective capacity increase. A critical objective of such architecture is to ensure energy efficiency. SEASON will rely on power-efficient Digital Signal Processing (DSP), MBoverSDM optical switching, point-to-multipoint transceivers allowing traffic aggregation/router bypassing, and converged packet-optical solutions reducing the number of O/E/O conversions. Such complex infrastructure requires rethinking the control and orchestration systems towards autonomous optical networks, addressing not only the integration - in overarching control systems - of the Radio Access Network (RAN), access and transport segments but also adopting more agile DevOps methodologies. SEASON will leverage on cognitive networks powered by streaming telemetry, real-time network measurements and Artificial Intelligence/Machine Learning (AI/ML)-aided service management and orchestration for near-real time network operation, moving intelligence as close as possible to the data plane, and devising a distributed system based on multiple communicating agents and data-driven closed control loops. SEASON will have a clear impact on the society, in a context with increased needs of connectivity and higher capacity demand required for services such as VR/AR. The SEASON consortium includes major European telecom operators (Telefonica, TIM), major vendors (ADVA, Infinera P/G, Ericsson), three consolidated SMEs (Accelleran, Wings and WestAquila) and four top-reputed research centres and academia (CNIT, CTTC, Fraunhofer HHI, and UPC).