Until Redroger arrives to the rescue; I suppose you've found the same 1921 description (below).
I'm inclined to think the daughter acted as the "pointsman or boy" who rode on engines with the driver.
A Dictionary of Occupational Terms Based on the Classification of Occupations used in the Census of Population, 1921
pointsman (or boy) (mines, quarries, works railways) ; latcher, latchman, loco latcher, shunter (mines, quarries, etc.)
rides on locomotive with driver, couples and uncouples wagons, and alters points on works railway or sidings to control movements of vehicles.
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pointsman, outdoor - see ground frameman.ground frameman, ground pointsman, ground signalman, groundsman ; outdoor pointsman
a pointsman q.v. who moves hand levers in the open to work single points or ground signals and points in ground frames.
http://doot.spub.co.uk/code.php?value=701In case it's of interest:-
t was not until 1871 that the first enduring trade union organisation emerged – the Amalgamated Society of Railway Servants of England, Ireland , Scotland and Wales (ASRS).
The United Pointsmen's and Signalmen's Society was founded in 1880 as the United Pointsmen and Signalmen's Mutual Aid and Sick Society. The name was shortened in 1908. The United Pointsmen's and Signalmen's Society was never a large organisation, when it amalgamated with the Amalgamated Society of Railway Servants of England, Ireland, Scotland and Wales and the General Railway Workers' Union in 1913 to form the National Union of Railwaymen it had just 4,101 members
The National Union of Railwaymen (NUR) was formed in 1913 by the amalgamation of the Amalgamated Society of Railway Servants of England, Ireland, Scotland and Wales, the General Railway Workers' Union and the United Pointsmen's and Signalmen's Society.