Presumably using TPWS and AWS in standalone mode rather than running under ETCS Level 0. Anyone know for sure?
Infrastructure isn’t there on the Mainline for ETCS - only on the Heathrow branch (and not yet signed off for passenger use there either otherwise we’d have 345s on the connects).
My understanding is that the Reduced Length Units (7-cars) are running an older build of software that only supports TPWS+AWS - it does not implement ETCS so cannot run in Heathrow tunnels. TPWS+AWS are not installed there and aren't going to be.
The Full Length Units (9-car) run ETCS as their primary signalling system, with a fallback to TPWS+AWS as Level NTC (National Traffic Control) in sections where ETCS is not installed: Reading to Paddington and Liverpool Street to Shenfield. This software isn't working reliably yet, so 9-cars can't run in service.
Originally the Crossrail project included installing ETCS on the Great Western Main Line as far as Reading. However that was descoped. It's now part of the general Digital Railway project and I don't think there's a set date.
Heathrow's tunnels may still depend on getting Heathrow Express stock upgraded for ETCS. Siemens are working on a first-in-class install on the 387s.
The third signalling system, used in the tunnel, is CBTC. This is actually a description, not a name - it stands for Communication-Based Train Control. I believe the system in use is Siemens Trainguard. I don't know whether this is their ETCS product running at Level 3 or a legacy one. To begin with, it was considered that ETCS L3 wasn't ready for this application, but Thameslink are now running that through their core.