Doesn't Occams Razor apply ?
i.e. the simplest theory is the most likely.
There may well be signal sighting issues ... although they can't be that bad if SN work 377s round there ... and until this thread I had never heard there were any anywhere.
But surely the reason FCC don't use 377/5s there is a lot simpler. The FCC loop services are Standard Class only, as are the shared FCC-SET stoppers to Sevenoaks etc, FCC Brighton services and the shared FCC-SET main line trains need First Class.
FCC have just about the right split of combined 319/0+319/3 to cover the S.C. only diagrams and combined 319/4+319/2+377/5 to cover those needing F.C.
They can't run 377/5 on loop trains without compromising F.C. where it is advertised and where it is probably a franchise commitment.
So not only do they not do it, despite cleared to do so, they don't have any need to now nor make a business case to sort out whatever these alleged "signal sighting" issues are, if they exist at all.
Yes I know there are peak variations, and FCC's ability of getting the right 319 on the right train is, shall we say, not good based observation, so 319/4etc do appear on the loop and 8car 319/3etc do go to Brighton, but none of that disproves the fact loop trains are SC only and to diagram 377/5s there would be a level of lunacy that even FCC does not possess.
Further, FCC has diagrams that can only be 377s that 319s may not work.
FCC 12car Thameslink trains cannot be 319s. The oft quoted reason is FCC has no safety case for 12car 319s but there is no point getting one anyway since under AC the combination of pantograph and OLE limits 12car 319 to 75 mph but service needs 100 mph
Also the SET FCC workings to Rochester, Ashford, etc must be 377s as either SET drivers on those workings are from depots that do not sign 319s or certain routes are not cleared for 319s.
Add up the 4 12car Brighton 377 trains and the (at least) 3 8car Kent 377 trains and youve used up 18 of the total 26 FCC 377s.
It all means FCC has no practical use for 377s on the loop without compromising something else
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Nick