Every ETCS conversation starts with "which baseline?" — and since the 2023 CCS TSI made Baseline 4 the mandated specification set, the question has a new default answer. Here is the comparison that matters for procurement, retrofit and trackside programmes.
What a baseline is
An ETCS baseline is a frozen, consistent set of the ERTMS specifications — chiefly the System Requirements Specification SUBSET-026 plus its interface and test companions — released together so equipment built to the same set interoperates by construction. 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. Baseline 4 is SRS 4.0.0.
What Baseline 4 brings
Three headline capabilities define B4. First, radio evolution: the specification set is written for the dual world of GSM-R and its successor FRMCS, with the interfaces (including SUBSET-037's FRMCS part) structured for migration — essential because GSM-R's industrial runway is finite. Second, ATO over ETCS: automatic train operation up to GoA2 enters the mandated set through SUBSET-125 and SUBSET-126 (both v1.0.0 in Annex A), turning what were national pilots into an interoperable function. Third, a broad sweep of clarifications and new functions accumulated since 3.6.0 — the reason SRS moved a full major version.
The legal anchor
The 2023 CCS TSI (Regulation (EU) 2023/1695, since amended on testing specifications) lists in its Annex A Table A 2 exactly which document versions are mandatory: SUBSET-026 at v4.0.0, with the companion subsets at their B4 versions — the per-document versions are shown on each subset's page in this library. New trackside and on-board developments are assessed against this set; the TSI's transitional provisions govern how long B3-based work may continue.
Compatibility: the part everyone asks about
B4 tracksides do not strand B3 trains. ETCS manages mixed fleets through system versions: a trackside announces the system version(s) it operates, and an on-board runs with any version it supports up to its own capability. A B4 trackside can therefore serve legacy B3 vehicles by operating compatible system versions, while B4 vehicles gain the new functions only where the trackside enables them. The practical consequences: fleet owners should plan on-board upgrades around natural mid-life events rather than emergency retrofits; trackside programmes specify B4 for new deployments; and testing strategies follow SUBSET-076 at the versions Annex A pins.
Who must care, in one line each
Vehicle owners: new authorisations land on B4 — check your on-board supplier's roadmap. Infrastructure managers: new ERTMS trackside is B4 territory, with FRMCS migration planning attached. Suppliers: dual-baseline support windows are now a product-strategy question. Everyone: the key management and train interface layers evolve with the baseline — budget the integration work, it is rarely the cheap part.