Can you solve tunnel puzzler?

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I was intrigued by Les Singleton’s recently published image of icicles on the roof of one of the Monsal trail tunnels (Mercury letters, March 7).

It reminded me of a puzzle I have been unable to solve since walking through the tunnels following reopening last year.

Can any reader explain the existence of the bricks attached to the walls and spaced at regular intervals on either side of each tunnel, about 4 metres above floor level?

Phil Askham

Matlock