David Monk-Steel has commented
The writing on the board comprises BR (pre TOPS) Routing codes so presumably it was there to inform shunters what traffic should be grouped with what.

Sample route codes include -
ASHFORD KT -SM3
TONBRIDGE - SM1
LONDON PLUMSTEAD - SJ1
LONDON WALWORTH ROAD - TR1
BASINGSTOKE - UI1
NORWOOD JUNCTION - TD1
BRIGHTON - TN2
SOUTHAMPTON BEVOIS PARK -UK7
SOUTHAMPTON DOCKS - UK9
WIMBLEDON - UA2
WOKING - UK2
EXETER - XL2
SEVERN TUNNEL JUNCTION - WD1
WESTBURY - XF1