A spokesman for the federal police headquarters in Bexbach, northeast of Saarbrücken, said that where possible every train was being searched.
"Officers are concentrating especially on surveilling train connections and air traffic," a federal police spokesman said in Potsdam earlier on Saturday.
Interior Minister Thomas de Maizière had earlier said that additional heavily-armed police would be sent to airports and train stations.
The state of Rhineland-Palatinate, which borders the French regions of Alsace and Lorraine, sent its own police to join federal officers in heavily-armed patrols along the frontier.
Its police helicopter squadron is also on high readiness.
Patrols have been issued armoured vests and submachine guns, a spokesman for the Rhineland-Palatinate interior ministry said.
He added that there would be additional "covert measures" introduced to secure the border.
But there was no specific indication of an attack targeted at the west German state.
Police in Saarland have also been checking arrivals by road from neighbouring French region Lorraine throughout the day.