Rotherwas Junction is situated just over a mile south of Hereford (Barrs Court) station where the 'main line' to Newport diverged from the line to Ross-on-Wye.
The documentation here is fairly complete and illustrates the position well. The frame was shortened from 78 to 54 levers in 1925 and I have included a hand drawn box diagram to show the position at this time.
The first change is shown on a 'locks on/off' notice dated September 14th 1928 detailing the alterations made when points 28 in the Up Sidings was converted to hand operation - shunting signals 23, 24, 35, 36, 37 and 49 were recovered and some replaced by stop lamps. Levers 7, 23, 24, 25, 28, 35, 36, 37 and 49 thus became spares. I have amended a later dog chart to show the locking provided in 1925 - the locks on/off notice and the erasure marks on the chart were sufficient to allow this and a locking table has been derived from this dog chart.
The dog chart dated 17 June 1929 shows a further alteration to provide a working Down Main Distant worked by a new lever 0, working outside the end casting, extending the frame to 55 levers - again a locking table has been derived from this dog chart.
The story is taken further by the amedment state page, S2248/1, the next change noted being in November 1952 (amendment 'A'), and the subsequent changes are shown below:
A - Nov 1952 - Down Main Distant renewed as C/L
B - Feb 1957 - Elbows recovered i/c/w engineer's relaying
C - July 1957 - step ports provided in 2 and 5 locks
D - June 1958 - Up Main Distant renewed C/L
E - August 1963 - Down Branch Distant renewed
F - October 1964 - Levers bolted i/c/w recovery of line between Rotherwas and Ross
G - June 1968 - crossover and up line signals recovered, down line interlocking lever provided
H - July 1966 - 'G' modified i/c/w retention of Up Starting Signal (renamed 9 and slotted by Aylstone Hill) and Distant FAC
J - July 1968 - Closure of Red Hill Junction SB
This shows a fairly major change in February 1957 and a locking sketch shows proposed signalling alterations in connection with relaying, involving the replacement of F.P.L. bars 20 and 45 (elbows) and 17, being superseded by new track circuits, lever 45 becoming a space and lever 25 being used for the Down Branch to Up Main trailing connection (ex 22C). The final 'locks on/off' notice stamped 4 March 1957 shows lever 45 being removed and some sequential locking being added. Lever 25 remains a space because the relaying has actually been as a single lead junction, with 22 becoming a disc signal.
A later locking sketch part 1 and part 2 show the position in June 1958 after the Up Main Distant had been renewed as a colour light - an early example where the double junction had already been replaced by a single lead junction. The locking table sheet 1 and sheet 2 (alternatively shown in 'html' form) and the dog chart (stamped 4 MAR 1957) show how the locking was implemented. The detection schedule, electrical control table and lever leads sheet 1 and sheet 2 complete the picture. The box diagram for 1949 is available from the Signalling Record Society.
A simple demonstration of how mechanical locking operates may be viewed - as an example we will look at a section of the locking at Rotherwas Junction from levers 14 to 23 (after the 1957 changes).
An engine has arrived from the Newport direction and is standing at no. 50 signal waiting to be shunted onto the Up Branch to allow trains to pass on the main line. The lever lead for signal no. 14 reminds the signalman of the necessary pulling sequence and we will watch the mechanical locking operate as each lever is moved - click here to see the diagrammatic 'dog chart' moving as a simple sequence of shunting takes place. Alternatively click here to see the actual locking being assembled and then moving as the same shunting manoeuvres are repeated.
The drawings for Rotherwas Junction show the layout after the double junction had been converted to a single lead junction in February 1957 (Yes, they did it even then) and was marked up to show the changes necessary for closure of the branch to Ross in October 1964.
The frame at Rotherwas Junction was a standard Great Western 3-bar 4" horizontal tappet frame. This is made up from standard locking trays with eleven channels. Each channel houses three bars - bars 1 and 3 above the locks and tappets and bar 2 below them with the majority of the locking on the top bars. Bar 2 is only drawn on the dog chart where it is actually provided but bars 1 and 3 and the outline of each tappet are always drawn. The nibs or locks are screwed to the bars and where a lock is fixed to a bar this shown by a dot and the ends of the bars are shown by a cross. The tappet moves 'downwards' (towards the back of the box) as the lever is reversed, the travel of the lever being converted by a cam so that half of the movement of the tappet occurs as the lever is being drawn out of the normal position in the first few degrees of movement and the remainder as the lever reaches the reverse position. Initial movement of the lever therefore immediately locks other levers and other levers are only released as it reaches the reverse position. The tappet travel is exactly the channel to channel separation so that a port acting as a 'normal' port in channel 1 will also act as a 'reverse' port in channel 2.
The locking chart (or 'dog' chart), on which I am focusing in this animation, shows the design of the mechanical locking to implement the locking table.