ETCS baseline

An ETCS baseline is a consistent, frozen set of the specifications that define the European Train Control System — chiefly the System Requirements Specification SUBSET-026 plus its companion interface and test subsets — released together so that trackside and on-board built to the same baseline are guaranteed to interoperate. Baseline 3 exists in two maintained states: Maintenance Release 1 (SRS 3.4.0) and Release 2 (SRS 3.6.0), the version most in-service fleets run today. Baseline 4 (SRS 4.0.0), mandated by the 2023 CCS TSI's Annex A, brings in FRMCS readiness, ATO over ETCS and other Game-Changer functions. Compatibility across baselines is managed through system-version negotiation: a Baseline 4 trackside can announce older system versions so legacy vehicles keep running, which is why 'which baseline?' is the first question in any ETCS procurement or retrofit conversation. The binding version numbers per document live in Annex A Table A 2 of the CCS TSI — reproduced on each subset's page in this library.