Melling Junction Layout
Approach, Back & Route Locking
Approach locking locks the route when a train is approaching a cleared signal. If the signal is put to stop before the train has passed the signal a timer must run down before the approach locking clears.
Back locking locks the route when the train has passed the signal.
Route locking is extended backlocking when a longer section of the route must be locked.
Approach and Back locking lock the signal, thus indirectly locking the points. Route locking (at least, at Petone) locks the points directly.
Clearing 14 signal extends 17 signal's approach lock to include 10T. If a train occupies 10T and 14 signal is put back to stop then the timer of 14 signal, as well as that of 17 signal, must run down before 17 signal unlocks and the points can be operated.
Why does 78T NOT route lock 47 points directly?
Consider a down Melling train. 78T will route lock 48 points reverse. In turn 48 points reverse will lock 47 points reverse, so 47 points are locked indirectly.
The reason for this indirect locking is to avoid delays when an up Upper Hutt train follows an up Melling. 48 points will be normal and so will not lock 47 points. The latter can be moved as soon as 47-48T is clear. If 78T locked 47 points directly then the points could not be operated until the up Melling train had passed 78 signal, a distance of about 500m.
It is sometimes suggested that it would be useful to remove the locking between 47 and 48 points. The above shows that doing so would create the possibility of a run through of 47 points.
Down Train Disabled at Ava
If a down train becomes disabled at Ava the signalman can "take the light back" i.e. put 77 signal to stop. 48 points will be approach locked and there will not be a timer to run down. If there is a down Melling service waiting at 78 signal there are two options - either handwind 48 points (20 minutes) or verbal the Melling onto the up main (48 points are locked normal, 47 points are free) and through 38 points at the south end.
Why Is 78 Approach Locked "When Operated"?
The Melling Branch is not completely track circuited so the system does not "know" where a train on the Branch is. Therefore if 78 signal is cleared and then put back to stop before a train has occupied 78T a 90 second timer will always run down.
Overlaps provide protection from an overrun signal. If 48 points are normal an overlap is available at 17 signal regardless of the position of 47 points. 47 points can therefore be operated independantly of 14 signal.
If 48 points are reversed then there is no overlap in advance of 17 signal so 14 signal must also be at stop, the 14 signal to 17 signal section being the overlap.
It is possible to signal a train past 14 signal with 48 normal, put 14 signal to stop, run down a timer and then reverse 48 points - handy if a down Melling is on the branch and an up Melling has been signalled up to 17 signal.
On the down 77 signal and 78 signal are about 500m from the points so an overlap is available.
What Are MBR And MBCR?
The locking table refers to MBR and MBCR. These may be Magnetic Block Relay and Magnetic Block Proving Relay or they may be Melling Branch Relay and Melling Branch Clear Relay. I do not know. These relays were probably installed in the 1980s when the signalling and most track circuits were removed from the Branch.
MBR detects the Branch clear (tracks and MBCR). Its state is shown by the Branch Occupied lights
MBCR is a latched relay with the following logic - with 47R and 48N a train entering the branch latches the relay to the "train entered branch" state. With 47R and 48R a train leaving the branch latches the relay to the "train departed branch" state. If there are any non-standard moves the Branch Occupied lights cannot be relied on.
Sometimes, if 47 and 48 are normal, a faulty train on the main drops 79T which turns the Branch Occupied lights on. Cycling 48 points clears 79T and turns off the Branch Occupied lights.
Last Updated: Thur Aug 8< 2013