The Spanish army is guarding rail links across Spain as part of a package of security measures introduced after the discovery of an unexploded bomb.
Spanish civil guards inspected the area after the device was found
Police found the device on a high-speed railway between Madrid and Seville on Friday.
The explosive content is said to be the same as that used in last month's Madrid train bombs, which killed 191.
Forty-five helicopters, sniffer dog teams and armoured vehicles are involved in the security operation.
The line has now reopened after intensive checks following the discovery of the bomb at Mocejon, in the Toledo area, about 60km (40 miles) south of Madrid.
Bomb disposal experts found between 10 and 12kg (22-24lb) of an explosive resembling dynamite on the line, Interior Minister Angel Acebes said.
The explosives were connected to a detonator with a 135m cable.
Mr Acebes said the bomb had failed to explode because it had no trigger - suggesting that those responsible may have been scared off by security guards as they were planting the bomb.
He added the explosive had "the colour and texture" of Goma 2 Eco - the Spanish-made dynamite used in the 11 March attacks on four commuter trains in the Spanish capital - and further analysis would tell if it was the same material.
The alert came as Spanish judge brought provisional charges against a 15th suspect in the Madrid bombings.
Otman El Gnaout, a Moroccan, is accused of collaboration with or membership of a terrorist organisation.
The judge also released four men on Friday, two of them Syrians, one a Moroccan and one a Spaniard.
The BBC's Katya Adler, in Madrid, says despite a call by Mr Acebes for people to avoid rushing to conclusions, much of the Spanish press and the general public have decided that Islamic extremists planted this latest bomb.
She says Spaniards have reacted in stunned disbelief at the news of another attempted attack.
Our correspondent adds that trains and motorways are packed across the country at this time of year as millions of families set off on their Easter holidays.
Several newspapers reported on Saturday that the Spanish embassy in Egypt had recently received a letter signed by the Abu Hafs al-Masri Brigades threatening to attack Spanish embassies and Spanish interests in north Africa and the southern and eastern Mediterranean region.
The letter warned that the attacks would go ahead unless Spain withdrew its troops from Iraq and Afghanistan within four weeks, El Mundo reported.
Last month's attacks in Madrid continue to have international repercussions.
Friday saw Nato foreign ministers agree a series of measures to fight terrorism, including more sharing of intelligence.
And in Washington, security officials warned that the Madrid bombings had increased the level of concern about a possible attack on US soil.
The FBI and the department of homeland security said extremist groups might try to bomb buses and rail lines in the US, hiding explosives in luggage and carry-on bags.