Sorry this post is a bit lengthy, I've tried to cover everything as I understand it, starting from the beginning, and I think I've got it right Hope it all makes sense!
Baker St was resignalled either late 1989 or by June 1990. Presumably this is when that section between 3B points and the approach to Regents Park SB platform was changed to Block jointless track circuits).
Looking at the changes to the signalling compared to as it was in the yellow peril (I'll call this OLD) and as it was from 1990 (I'll call this NEW):
BM500 - platform draw-up - removedA399 - replaced by A4011 (in a similar position)
R397AB - removedR397BC / A397A - becomes A395
A397B - removed
A397C - removedA395 is the only home signal for Regents Park SB
This is followed by A391 which is the SB starter for Regents Park.
Thus, between Baker St inner home and Regents Park SB (starter to starter), there are the following signals:
OLD signalling - at time of yellow peril:
A401 (not shown on YP) (Baker St plat 8 outer home), or possibly A401A (outer) and A401B (intermediate)?
RBM500 / BM4/400 (inner home)
BM500 (platform draw-up
BM5 (starter)
A399
A3991
R307AB
R307BC / A397A
not shown on YP clip, but presumably followed by:
A397B
A397C
A391 (Regents Park starter)
NEW signalling - 1990:
A425 (outer home Baker St plat 8)
BM4 (inner home)
BM5 (starter)
A4011 / R395
A395 (home)
A391 (Regents Park starter)
As I see it, under the OLD system, BM500 would act as a draw-up signal that protected 3 crossover. Presumably BM500 would go to yellow after X seconds and thus an approaching train would either:
1 have slowed down (to perhaps 10 or 15mph) and so reduced the train's braking distance. BM5 would then be protecting the crossover
or
2 would be going too fast and so reaching BM500 before it went to yellow and so tripping the train.
This is the standard method used where it is desired to be able to allow a train into the platform when the crossover is reversed - i.e. two trains can be travelling in the same direction at the same time. This saves time over the other method of holding a train outside the platform while a train crosses over from the other platform or waiting until the approaching train has stopping in the platform before allowing the train from the other platform to proceed.
This was common at Kennington SB City where B800 in the platform slowed the train because the points ahead were set for a SB Charing Cross train towards Morden.
(B800 was under the platform ledge and poorly sighted with no repeater. In days of old, it was common for drivers not to notice it was red and get tripped at it. A suitable choice of words was uttered by the driver before they got down on the track to reset the trip and carried on. Golders Green NB has a similar layout, but it was worse for the driver then because a driver getting tripped was usually greeted by a rousing cheer from other crews on the platform
)
Under the NEW system, BM4 protects the crossover and will only clear if 3B points are normal. BM5 is approach-controlled and will not clear until a train has occupied the platform track for 15 seconds. If there is a train in the platform, BM5 will be protecting the crossover.
This is another common method used and examples on the Northern line were Moorgate NB and Kings Cross NB. An approaching NB train would either be held outside the station while the train ahead was reversing, or the train would be held in the platform and once it was detected that it had stopped, the crossover ahead would be reversed.
A4011 must be green before either starter BM2 (route 2 to cross from Jubilee to Bakerloo line) or BM5 can be cleared. This effectively means that the track ahead must be clear as far as the overlap of A395 - the start of the Regents Park platform track circuit. I.e., if following a train, BM5 will not clear until the train ahead has berthed in Regents Park platform.
As far as I can work out, A4011 serves no real purpose as far as track capacity is concerned and would make no difference if it wasn't there, other than being one additional stop signal for the purpose of applying the rule. A4011 should never be seen at danger by an approaching train unless there is a signal failure.
Assuming that A3991 on the OLD signalling was a round the bend signal, I assume that it still serves the same purpose now that it has been renumbered to A4011.
What is a little strange is why they renamed the signal to A4011 when there was no longer a 401 signal in use. Perhaps they just liked the idea of keeping it four digits, but why not leave it as A3991?