"Capacitors" on the Central Line diagram(s)

Well they represented the fact that these were termini for most services. So Hainault is the clearest-cut example. The Hainault to Woodford service was a self-contained shuttle, trains didn't run Grange Hill - Hainault - Fairlop or vice versa. And again, similarly, all trains terminated at Hainault via Newbury Park, end of the line, no carrying on to Grange Hill. I mean, there may have been some exceptions, but the main point is to show Hainault as basically the end of the line for most purposes. Whereas now, of course, three trains per hour do carry on.

Similarly Epping was the terminus of the line for most services. Now I know when the Epping Ongar service was relegated to peak hours only, the unit made its way up to Epping from Loughton and I think it may even have been in service, but it's to show people that basically you're gonna be on an Epping train and there's no point waiting at Oxford Circus for that Ongar through train, because it ain't coming.

There were various ways the situation could have been presented, such as two linked interchange blobs, ( as at Woodford) or even showing the shuttles in a different colour to the main Central Line, but the designers of the map clearly decided this was the clearest way of showing that there was no through service.

The same convention has been used at Ormskirk, Kirkby, and, when they too were the limit of electrification, Royston and Rock Ferry

Kirkby:





Ormskirk:





and this rather nice old pic. The "Emergency Connection" in the foreground has now gone:

As the platform at Epping is only long enough for one train, the Ongar shuttle actually used the other platform, so the trains were side by side, rather than the more convenient end-on arrangement used at Ormskirk and Kirkby.

As to why they did it, the extension of electrification to Ongar was done on the cheap, and couldn't cope with eight car trains.
The Hainault shuttle was used as a test bed for ATO, which meant operations on the branch had to be self contained.

It was indeed in service and there was a time where the shuttle stabled at Hainault off-peak, hence the ability to catch a through train from Woodford to Ongar (which I did on occasion, and has come up on another thread recently as well). The tracks at both Hainault and Epping always allowed through running and there were no end to end buffer stops which surprised me the first time I saw them as, like 411, I was anticipating trains end to end. Quite how they'd have got the shuttle onto the section, that being the case, didn't occur to me at the time as I was only 5 or 6.

An advantage of Pro-Boards, then. You can't (well, couldn't the last time I checked) do that on the more common Jelsoft forum software.