Melling Branch Circuits

In 1983 most signalling and track circuits were removed from the Melling Branch. Since then trains have been signalled on a train on/train off basis. When there are non-standard moves the system is not foolproof and the signalman must pay careful attention to the train register. On this page I explain the ciruits used to signal trains on and off the Melling Branch, in particular the Melling Branch Proving Relay (MBCR).

Panel Indications

Panel Indications

Track Indications. The indications for 47.48T, 78T, 79T operate in the usual way.

MBTXR is a rail circuit that proves the presence of a train, unlike a track circuit that proves the absence of a train.

A and B are illuminated when one or more of 47.48T (with 47 points reverse), 78T, 79T is occupied or relay MBCR is in the Train On Branch state. When a train is in the non-track circuited section no track circuit lights will be on but A and B will be illuminated if the MBCR relay is in the Train On Branch state.

Non-standard moves can result in MBCR in the "wrong" state.

Melling Branch Proving Relay Circuits

MBR Circuits

MBCR. If 47 points are reversed and 48 points normal a train occupying 47.48T (with 79T not occupied) will put MBCR into the Train On Branch state.

If 47 & 48 points are reverse a train occupying 79T (with 47.48T not occupied) and then passing 78 signal and operating MBTXR (which proves the presence of a train) will put MBCR into the Train Off Branch state.

Branch Clear. The Branch is clear (MBR) if 79T, 78T and 47.48T (with 47 points reverse) are not occupied and MBCR is in the Train Off Branch state.

Indications. In addition to the usual panel indications for 79T, 78T and 47.48T there are two sets of lights that indicate the Branch occupied. These are controlled by MBKR.

Wrong Side Failure. If bonds for 79T are disconnected the track relay will be dropped. A train verballed onto the branch will not operate the MBCR Release coil and so MBCR will remain in the Train Off state. This may be the cause of an observed wrong side failure in which detection of a train on the branch was lost.

Clearing 17 Signal

Clearing 17 signal

48 Points. These are trailing so must be Normal.

47 points. If 47 points are Reverse then both MBR and MBCR must prove that the Branch is clear.

78 Signal. There is no contact for 78 signal normal because 48 points normal ensures this condition.

Switchout. When the up main is switched out the switch for 17 signal on the panel is bypassed.

Last Updated: Tue Jun 5 15:43:16 NZST 2012

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