Paul Corfield wrote:
There may also be something to do with the type of track installation as
well - ballasted vs concrete track bed - but that's getting beyond my
limited knowledge.

The latter type is more commonly referred to as "slab track", as that
is what it literally is: a slab of concrete with rails on it.

The actual track feature itself is referred to as an "expansion
switch", and does what it says on the tin: allows the rails to expand
and contract as they heat and cool. They are quite commonly used in
areas with unstressed welded rail (UWR) or long welded rail (LWR) - if
I remember correctly, continuous welded rail (CWR) only uses them in
areas where pointwork is present; the former two generally use them in
various spots when needed.

The ones on the DLR come in two varieties: the normal "slot" variety,
where the rails are cut into a yin/yang-type slot arrangement and
anchored together, and the "slip" variety, where one rail curves
outward to gauge and another is trimmed to butt up against it snugly
like a point blade.