I assume that's because the Midland Railway were prioritizing Derby rather than London, unlike some of its contemporaries.
The Midland didn't prioritise Derby for the purposes of either milepost zero points or the selection of up and down directions, at least not after all the mileposts were re-positioned in the early 20th century.
As explained by the article below, the Midland implemented a logical scheme for defining its mileposts starting from a zero at St Pancras. Derby to St Pancras is still the up direction and the direction of decreasing mileage.
Confusion may arise because branches which diverged from the direction of increasing mileage were measured from a zero at their junctions, and the most famous of these was from Derby to Bristol and Bath. This route was also "up" towards Derby, possibly because departures from Derby towards London were originally from the north end of the station, but this arrangement was convenient as directions lined up with the LNWR at New Street and the GWR in the Bristol area (though not the parallel GWR tracks between Gloucester and Standish).
http://www.midlandrailway.org.uk/occasional-papers/midland-mile-posts/
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On the Western we had lineside plug points which were an access to an ETD line, we just carried a small phone handset and literally plugged it in to use, we recovered most of those as most staff have mobile phones now, it could be a similar thing. I suspect the cabinet is a Telecom Distribution cupboard and contains comms cables, if so it'll have a ridged roof and a long number on a (yellow) plate just above the door but lots of kit is found in similar cabinets. Hope this helps.
Seen those on other regions too. They were orange sockets with a spring-loaded lid, externally a bit like those shielded 110V sockets that builders use, labelled "Plug Point" but this would probably be too small to see from any distance.