Likewise "operational separation" of different lines is key to reliability, both to ensure incidents on one line don't affect service on another, and because points are subject to a lot more wear and tear than plain track (see the current disruption to south London rail services, caused by a cracked crossover at Lewisham). Going forward TfL is trying to move towards a network of simple, self-contained end-to-end lines, e.g. by separating the Northern line into two independent lines rather than one line with five branches.
The kind of thing you're describing is in operation elsewhere in London where the capacity requirements make it more appropriate: http://en.wikipedia.org/wiki/ULTra_(rapid_transit)
