"Black" signals (for want of a better description)

As I understand it, a signal with no aspect illuminated must be treated as if it was at danger. The same applies if it shows two aspects (e.g., red and green), or if it is bobbing (i.e., aspects changing back and forth between red and green). I'm sure that I read somewhere that low air main pressure can cause bobbing signals, due to the trainstop going up and down - can someone confirm?

"Should" being the main word there... not always possible to do so! I have passed a signal with no aspect at almost line speed before, solely as it was the green aspect out, so the repeater was fine! As i came round the corner, nothing showing, but it was far too late to stop! Obviously if the signal in question was at danger though, the repeater would be on, so no excuses for hitting it really!

I completely agree there. What, if anything, is (or should be) announced in that situation?

I have as yet only ever been on one train that passed a signal at danger under rule.

As does the 95TS.

Yes, as otherwise it becomes an aggravated SPAD (and that's not something you want to be involved in).

And then there is "tripped in vicinity"!!!!!

Not a tripcock tester (no worries on getting tripped on a tester, so long as the light goes out!).

Its getting tripped and not believing it to be a signal. It (some say ) has to be treated from a SPAD point of view, until confirmed otherwise.

Plastic bags, high ballast, footballs or lumps of wood are the usual suspects ;D ;D

It is possible to be tripped on a tripcock tester if it is out of alignment but it would need to have sustained an impact for that to happen as they are routinely gauged and tested! It happened several times at Wood Green E/B a few years ago and I changed the tripcock tester on that road twice within a matter of weeks because of it even though the TT was in gauge. I believe we suspected a fault with the tripcocks being out of alignment on some sets at the time but the sets in question weren't being tripped on the TTs at Acton.

'Tripped in vicinity' can be the result of all sorts of things left on the track or lobbed there by vandals or indeed such things as pickup shoes knocked off earlier trains and yes high ballast is a culprit. Ballast is notorious for causing trainstop failures to first trains following rerailing too, where it gets caught preventing the arm coming to the fully 'on' position.

Other causes of 'trips' are such things as tin cans blown into the trainstop can guard (one type known as a 'Roman helmet') by the passage of trains and preventing the trainstop arm being fully lowered. Can guards are fitted to trainstops in tube tunnel areas but seem to attract rubbish rather than deflect it.

Suppressed Reds can be a nightmare for the new lineman, I reckon every lineman gets caught once somewhere! The one not to get caught on is the one in the T4 loop. It is suppressed because of the extremely long and very straight approach which might cause the driver to slow from normal line speed from too far away and thus delay the service unnecessarily. If you think it's failed it's a long walk back to check the bulb only to find there's no circuit until a train is on the approach track circuit!

Bobbing is caused most often by the section rail (which is the feed side of a track circuit) of one of the tracks in the signal selection being intermittently earthed. Causes of such earthing are scaling of railheads contacting earth, or discarded drinks tins bridging the rail to an earth etc or the breaking down of a blockjoint by the building up of magnetised iron filings around the insulating 'T' piece which keeps the rails of two track circuits apart. Obviously there are other possibilities associated with wire and equipment failure which I won't go into.

From Picadilly Pilot's tales of the past on DD's main site:
A DR train got tripped and caused a bit of delay (take the comments such as "how would anyone notice" etc. as read). (Readers - please note: - It's been Standard Practice for many years for there to be more than a degree of 'banter' between Piccadilly and District Line Train Crew. One overworked joke is that the Picc has a timetable - the District just works to a calendar! District Dave)
When the Motorman explained the cause to the Regulator he was disbelieved. A few hours later the door of Earl's Court Regulating Room flew open, the Motorman standing there asked who the Regulator was he'd spoken to earlier. He then went over to the Regulator's desk, slapped down the dead pigeon he'd been tripped on and said, "Don't call me a liar".