The associated box diagram is available from the Signalling Record Society.
The drawings for Morlais Junction were produced in April 1970 when some modifications were made to the layout but include the dog chart and locking table as at November 1965. This is another example of GW 3-bar locking albeit a bit simpler than Rotherwas Junction and perhaps a little easier to understand. The drawings were produced when the connection from the Up Branch to Morlais Colliery was put onto a ground frame but includes earlier versions of the drawings when interlinking and sequential locking were added and line clear releases were provided in 1965. Llangennech Station and Sidings signalboxes were taken out of use and a ground frame provided at the same time. The box then worked to Llanelly (Llandilo Junction) in the down direction and to Pontardulais (Hendy Junction) on the Up Branch and to Skewen (Grovesend Colliery Loop) on the Up Main.
The changes made when in June 1961 the fouling bars on FPL 14 were removed in connection with relaying and a track circuit 14T provided are also recorded. It thus showed some locking modifications, included previous versions of affected drawings and described electrical modifications in connection with improved block controls. The block controls were untypical as the box was permitted to switch out either with the route set for the Up Main (i.e. for the goods lines around Swansea through Skewen) or for the Up Branch (i.e. for the Central Wales line). Grovesend Colliery Loop was at the northern end of the triangular junction and some interesting electrical controls were provided because the double junction it controlled was in rear of its Up Distant (i.e. in the block section) and the junction wa therefore protected in the trailing direction by an Interlocking lever and electrical controls on the block instrument and the Morlais Junction Up Main Starting and the Hendy Junction Up Loop Advanced Starting signals.
The drawings for Morlais Junction include the dog chart (drawing S2115/6) as at November 1965 and I have made a locking sketch of the connections as they would have been at that time based on the sketch showing the layout after some connections had been removed. The locking table, drawing S2115/3, shows the locking functions that were provided. Each entry in the locking table can be found on the dog chart so that, for example, 4 released by 9 can be found on bar 1 in channel 1.