I suspect most people on here realise the type of train the OP was discussing are long gone. So what do we do ? , say no and lock the thread after 1 reply,
EXCEPT
I will answer with two current examples.
LU operate trains for employees between (1) Seven Sisters and Northumberland Park depot, and (2) (far fewer) between Stonebridge Park station and Stonebridge Park depot; these may be found in the LU WTT, in the public domain, even if public travel is not allowed on them.
Yes they are "staff" trains - the OP stated "employees of major business" but staff are employees and LU is not a minor business.
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Works halts that come to mind are British Steel Redcar, IBM in Scotland, Stanlow - closed, mothballed and not in use respectively!
works halts / stations yes, but surely, at least in recent times like this century, were not those served by ordinary public passenger services, not employee specific trains.
In the same halt category was Ampress on the Lymington line.
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A large now long gone example was Singer Works west of Glasgow short branch off the main route; that one even got to be 25 kV. ok ok it might have been 6.25 kV - whatever it was it was OLE AC electrified and served by AM3 units.
Before AC :
A view of the Singer Works station in Clydebank.
www.flickr.com
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Technically run for employees but available for the public were the Clapham Junction / Kensington Olympia peak hours only trains - these were for Post Office Savings Bank staff at Blyth House. Specific trains yes, but they were public. Obviously in more recent years the route morphed into what it is now.