Al-Qaida is planning attacks on Europe's high-speed rail network, according to US intelligence sources.
The National Security Agency (NSA) warned its European counterparts that the group could plant explosives on trains and tunnels or sabotage tracks and electrical cabling, German tabloid Bild reported.
German authorities reportedly stepped up security and deployed plainclothes police officers at key stations.
The newspaper said that attacks on European trains were a "central topic" in a conference call between al-Qaida top operatives.
Earlier in August, Washington issued a worldwide alert about a high risk of terror attacks after intercepting a phone conversation between al-Qaida leader Ayman al-Zawahiri and Nasser al-Wahishi, the head of the terrorist group's Yemeni brench Al-Qaida in the Arabian Peninsula (Aqap).
Twenty US diplomatic missions in North Africa and the Middle East were shut down on security grounds and a number of other diplomatic compounds representing Western nations were closed. The UK temporarily withdrew all staff from its Yemen embassy.
It was later revealed that another 20 al-Qaida operatives were in on the call.
The New York Times quoted an American official briefed on the intelligence report who said: "This was significant because it was the big guys talking, and talking about very specific timing for an attack or attacks."
In 2004 almost 200 people were killed in coordinated bomb attacks on commuter trains in Madrid.
The attack was allegedly carried out by al-Qaida-inspired terrorist cells but no evidence was found.