In message , Clive D. W. Feather
writes

This is more obvious on the Central Line; you can see the block
boundaries along the platform, marked by a white board carrying a red
diagonal stripe. In both systems the train's speed is controlled to
ensure it will stop before hitting the previous train.
This is not new. Before the 70s we had 5 signals approaching a
station, the first released when you were down to 25, the second when
you were down to 20 and the next three released as the previous train
left the platform, with the last one releasing when you had a car's
length inside the platform and the original train still had a car in the
starters overlap.
This was common on both the Central and Northern lines.
The others I didn't work on and cannot therefore comment on.
--
Clive