In article , Bob Watt
writes
At least two locations on LU have switches which may well be unique to LU,
inasmuch as they can be set with both blades open, when they act as
catchpoints.

There's far more than two of these.

These are used where neither direction is really safe for "trapping" a
train that passes the protecting signal; the commonest location is a
central siding between running lines. Each blade is interlocked with the
points on the corresponding running line:

----------------N-------------------------
\ NORTHBOUND
----------------n-X-----------------------
\ \
\ \
\ S--------------------
X
/ N--------------------
/ / E
/ /
----------------s-X-----------------------
/ SOUTHBOUND
----------------S-------------------------

In normal circumstances the four blades N and S are open and the two
blades n and s are closed. Trains can proceed on the running lines. A
train passing the exit signal E will derail *in line with the track* as
it falls into the gap between the diverging stock rails.

To allow a train into or out of the siding from the northbound line,
points N are moved so that blades N are both closed and blade n is open.
Similarly for the southbound.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Written on my laptop; please observe the Reply-To address