An interesting point in the Mott MacDonald options report https://orr.gov.uk/__data/assets/pd...te-train-protection-options-review-report.pdf is the qouted date of 2028 for ETCS L2 fitment on Chilterns. That is tantalisingly close to the 2027 date suggested for completion of enhanced TPWS, so assuming the ETCS date hasn't also slipped, The TPWS contractor would be just tidying up on site as the ETCS contractor is arriving behind them to rip all the work out again. I might tactfully suggest that doesn't sound like the best value strategy available! I suspect that this COULD all pan out like GWML where various strategies were mooted, but in the end they just went straight for the signals retained ETCS overlay which doesn't require comprehensive resignalling as it can be coupled up to the pre-existing SSIs in various ways. I note from the GWML project description that Alstom are using switched balises, which strongly suggests they are actually implementing a hybrid L1/L2 scheme that may not use continuous movement authority refresh via radio everywhere, possibly reserving it for busy junction approaches for better aspect update where it can make a performance impact (and replicate the effect of the fill-in beacons of the GW system). These techniques could probably also find practical application on Chilterns and would represent a long term solution, not a short term 'bodge'. The problem remaining is the rolling stock, however. GWML has more modern fleet profie, all already ETCS enabled or 'ready'. The Chiltern fleet will need equipping and varies in its 'readiness'. Parallel operation of Selcab and ETCS on the same surface tracks during a changeover could be an option but would need some study to see if a phased implementation across the fleet might be possible, as the MM report also highlights the difficulty accomodating the Selcab and ETCS equipment on the rolling stock simultaneously due to space constraints. Dual operation of the ACEC equipment and ETCS in the Heathrow tunnels was a major problem, but that was a specific issue about duplicated transponders provided in the ACEC system for reliability, an unusually high signal level from the obsolete ACEC equipment at exactly the same carrier frequency as ETCS, and a specific 'waveguide' effect found in the tunnel environment that meant the ACEC signal propagated over a very wide area and interfered with or overwhelmed the ETCS balise signal, causing dropouts resulting in failsafe (i.e. stopping) responses. ACEC transponders and ETCS balises worked fine together on GWML surface sections however.