Our halt at Marshbrook has proved a little longer than I expected. After I put that post up Nigel Bishop sent me a link to his own nighttime photograph of the signal box and pointed out that Wikipedia claims it is the oldest surviving box on the railway network that is still operating.
I can't find confirmation of this claim, but I did come across Railway Signal Boxes: A Review, which was written by John Minnis and published by English Heritage.
The LNWR/GWR line across the Welsh marches was interlocked in the early 1870s and five of the squat hipped roof boxes, which are notable for being deeper than they are wide, survive. None are listed. Marsh Brook (1872) is the best surviving example of what is one of the largest groups of very early signal boxes to remain in operational use.So it sounds as though it is, at the very least, one of the oldest boxes still operating in Britain.
Note, incidentally, that the box is called 'Marsh Brook' while the village, at least these days, is always written as 'Marshbrook'.