Mechanical signal replacer

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John Ashworth
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Joined: 24 Jan 2007, 14:38
Location: Nairobi, Kenya

Mechanical signal replacer

Post by John Ashworth »

There's an interesting discussion, with photos, on the sar-L of the "mechanical replacer". Apparently it was a device which returned the signal to the on position as soon as the locomotive had passed it.
I understand the need for such a device but its necessity leading into the crossing loop at Kloofeind from the Kimberley side always had me scratching my head.

It was such a hard slog up that grade from De Brug that trains were mostly down to a slow crawl that the replacer was triggered by the front wheels of the engine bringing the signal down with such a distinctive 'crash' often just above the cab (then you knew you were slow...). So much so that it often made me jump even above the sound of the stoker.

On such a steep grade with trains not travelling all that fast uphill (and barking in full roar) I figured that the signalman would have had the signals back before the rear of the train had cleared the signal post. But, I guess (conditions notwithstanding) the technicality of another train being 12 minutes behind anywhere on the line, the 'regulations' had to be adhered to.

I was so fascinated with this mechanism that I made a special trip to Kloofeind to photograph this mechanism and I will now try to attach my two (scanned) shots at this location.

The first photo is a double header from Kimberley lead by 3440 approaching the dual outerhome/distant pair leading into Kloofeind (with only the outer home in the 'off' position) meaning a cross with another train was to take place in the loop.

The second shot is of the actual replacer mechanism itself (a couple of metres behind the signal so out of view in the first shot).

Note the flange depression plate in the flangeway, just visible on the LHS with the release pins under cover on the RHS. I now regret not taking a second shot a bit futher back as it looks all too close up when I look at it now.
Mechanical Replacers were generally used under the following conditions and, but for one exception I know of, never on Home Signals:
a) On Single lines where Permissive Trainsworking was allowed. Under Permissive Trainsworking rules 5 trains were allowed to follow one another in the same direction with 12 minute intervals. The last train in the series carried the tablet in order to allow the receiving station to clear the line after it's passage. A Mechanical Replacer was installed at the Outerhome and Distant signals to replace the signals to danger and caution respectively after each train has passed it.
b) On Single Lines where the signal post for the Outerhome and Distant Signals was not visible from the Signal cabin.
c) On Double lines where Tyres Block Instruments were used and the Advance Starting Signal (Section Entering Signal) had to be replaced to danger immediately after the locomotive has passed it.
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Christopher King
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Joined: 24 Jun 2010, 08:11
Location: Glenroy Australia (near Melbourne)

Re: Mechanical signal replacer

Post by Christopher King »

My recollection of them was on the Cape suburban lines. Looking back to the 50s I thought it was a great thing to replace the replacr when colour light signalling came in.
On the main single lines the replacer always replaced the outer home signal The distant went back to stop due to that actions of the drop of slot mechanism so the distant could not clear unless the outer home was clear.
It operated by a ratchet connected to the signal box and the pawl was connected to the signal The action of the train depressing the treadle lifted the pawl bar and the signal went to stop by its own weight. Usually two wires were used to restore the ratchet but in some locations I have seen a single wire with a weight on it to pull the wire back when the lever was replaced

I do regret now having been so keen to get the replacers out!
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