Police have arrested a man they suspect of stealing large amounts of high quality metal cable from the high-speed AVE rail track that will link Murcia and Almeria.
Thieves broke through concrete blocks that had been deliberately placed at the entrance to tunnels in an attempt to thwart them and the loss of expensive materials from the line.
Officers were alerted when private security guards, retained to protect the tunnels, warned they had surprised two people inside one of them.
The damage has included the theft of metallic piping intended for putting out a fire if it should start inside a tunnel.
The suspect, a 24-year-old man, is said to come from Roquetas de Mar. It is not known if he is involved with three other men who are also being investigated on suspicion of theft.
The value of the materials stolen is not yet known, but a police spokesman said the issue of cable theft was a growing problem in Spain.
In 2015, copper cabling was taken more than a dozen times from Adif facilities in Catalonia.
This is said to have caused a series of delays and shutdowns on the lines affecting more than 100,000 passengers.
Last year a Guardia Civil operation helped smash a gang who had been operating in several different provinces.
A total of 30,000 metres of high quality cable was stolen from rail lines and tunnels causing a financial loss put at more than €800,000.