A WW II Building.? If so, of what use...

appears to be over a water course of some description

A possibility:

Re: 20th Century Air-Raid Precaution Railway Control Centres. Available as a download to ARG members

www.airfieldresearchgroup.org.uk/members...grn-no-56-railway-cc

Plate 7, page 22 depicts an entrance to a semi-underground air-raid shelter on the main line near Peterborough

Plate 8, page 32 shows another one. It is entitled 'One of several entrances to the air-raid complex at The Grove, Watford.

There is a similarity to the structure in the OP.

This 1935 OS 2500 map shows the Goods Yard. Note that to the right hand side is a twolane feature probably a culvert

The unidentified structure is located (on this map) just below the letter 'E' in FRIMLEY ROAD

Courtesy of Old Maps website:



EDIT:

I'm unsure of the symbology on this map. Note also that opposite the assumed end of the feature, on the north side of Frimley Street is a continuation running southwest then northwest showed on modern maps as a water feature (open drain?)

Looks to me very likely that the structure is access to the below-street culvert piping water from the northern ditch southwards.

OS Map experts, over to you!

See water feature here:

wtp2.appspot.com/wheresthepath.htm?lat=5...grat=None&rgrat=None

Thanks Martin, I do have symbol guides but I am chasing something else so will leave it to the more knowledgeable to identify 'culvert'

If the structure is above the culverted stream might it not be a well house orhoused a pump or have been a water access point for fire fighting?