Stone

The small child in me can't resist a snigger when I pass this bit of graffiti in the Innocent Tunnel.

The Innocent is another old railway in Edinburgh, apparently so called because it was used by horse drawn carriages and not noisy dangerous steam trains. A quick google has also brought up the claim that no one was killed during its construction but given how many people died constructing things in "the good old days" this seems unlikely to me!