svr(eh)[22] - TPO ; interior of 80300

TPO letter sorting racks [the steps are for youngsters, like the group that have just left]
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TPO was a Travelling Post Office, used by the Royal Mail staff to sort letters and small packets whilst in transit between major towns and cities. Included picking up and setting down mail bags when on the move.
See also the short film "Night Mail" and the eponymous W.H.Auden poem ...