Could it be a disabilty aid?
it is only on the flat section, not the steep parts where an additional rail might be useful!
Disability aid fitted by workmen with a cruel sense of humour?
You don't get updates.
Also its earthed. Weird?
Probably earthed because of regulations Alice, with 25kV AC a few metres below a freestanding unearthed long length of bare metal could be dangerous, especially in damp air.
Railings like these were installed on the pedestrian railway bridge in Welwyn Garden City up until a few months ago. Only removed when the bridge was strengthened with additional props. So does Harringay Bridge need strengthening?
The issue will be corrosion below the deck. I noticed on Friday parts had been resurfaced I presume to stop water ingress to the underside. Any sign of cold and they lob salt all over it.