To reiterate what m’learned colleague said up thread - 142/143 getting ETCS is “old gen”. It was in the plans at one stage, but no longer, or at least not within the foreseeable future. The 12 silver sets are the fleet.
At present authorisation is only for 8 car operations under ETCS L2, we can do 4/8/12 under “National Train Control” - AWS/TPWS, but we cannot run under NTC on the Heathrow Branch except to clear the line of failure, at 40mph to the end of cab signalling boards at the crest of the flyover then forward as normal.
With the current 2tph service, the timings don’t work at all for stepping up, so if a circuit becomes uncovered, it is usually cancelled until a replacement unit can be sourced - the delays were manageable in 332 days but until the drivers are a lot more familiar with the 387s it’ll be unrecoverable with the time taken to set up and so forth. When it eventually goes back to 4tph then stepping up “works” at T5, with a driver change at T5. It does not work at Padd.
GW-ATP is still live on the branch, as are conventional lineside signals, and likely will for some time. On paper the 332s could still make an appearance, they’ve worked their official last train, but 6 units remain available for traffic. There are.... politics involved that mean it’s unlikely we’d see them though.