The Surrey Iron Railway may predate the steam locomotive, but it is not the world's earliest railway. That honourable title probably belongs to a railway which is not in the British Isles, not on the mainland of Europe, and not above ground.
Thasos is the northernmost island in the Aegean, and the only one belonging to the Greek province of Macedonia. It is very beautiful and famous for the quality of its white marble, prized by sculptors since ancient times. It also produces excellent honey, very pleasant olives (which EEC regulations make it difficult to buy in England), timber and a variety of minerals.
It used to produce gold. The mines are worked out now, but they lead us towards our railway.
During the seventh century BC, Thasos was controlled by the Parians, an Ionian people from Paros. They seem to have been the first systematically to mine the gold which seems to have been plentiful: Thasos was once known as Chryse.
In the last century, archaeologists began to excavate the early mines. They found, somewhat to their surprise, I understand, that the mines were quite extensive with long tunnels extending underground. They were so long, in fact, that it seems to have been impracticable to carry the ore back to the surface. Instead, a railway was constructed on which trucks could be pulled or pushed.
When found, the rails were still in good condition. This was not really surprising as they were made of solid marble.
I don't know whether this is the world's oldest railway, but I should think it's a good contender for the title – unless, of course anyone reading this knows better.
(Information from a local archaeologist met on Thasos last Summer.)