I think it's what they call in the trade a 'Power Pillar'. Used for all sorts of purposes.
As this one is marked 76 with modern stick on numbers, I think it's still active, or has been until recently. Might be revealing to find boxes 74 and 75.
The modern marking destroys any association with trams, canals or trolley buses. I vote for Plasma's street lighting suggestion. Why not open the door and look inside? I think the box will contain the master fuse / isolator for a line of street lamps. Other possibilities: anything consuming power outdoors needing a maintenance point. Belisha beacons, traffic lights, railway signals, speed cameras, telecoms, fire alarms, time clocks etc.
The obvious age of Plasma's cast-iron example rules out Fibre-optic networking and electric car recharging but such are to be seen in town centres.
I don't see Qualters and Smith in the characters, maybe Sulli … Smith Ltd, and the second line 'Old Foundry …..' Graces Gulde doesn't list any likely Sullivans.
Dave