In just over a year, Thunberg has inspired younger protesters in a global movement demanding action to slow the atmospheric warming that climate scientists say could ultimately endanger the survival of industrial societies.
Stepping off the night train Lusitania at Chamartin train station in the Spanish capital, the 16-year-old Swede completed a hastily arranged expedition, including a 21-day catamaran voyage across the Atlantic, to a U.N. conference originally planned in Santiago, Chile.
Thunberg was met by a crowd of reporters but did not speak as she left the train.
“I successfully managed to sneak into Madrid this morning! I don’t think anyone saw me … Anyway it’s great to be in Spain!” Thunberg tweeted after leaving the station.
The Lusitania, which runs daily, is the only direct train link between Lisbon and Madrid and takes nine hours to travel overnight between the capitals.
The annual summit kicked off on Monday with a call from U.N. chief Antonio Guterres not to be the “generation … that fiddled while the planet burned”.
Thunberg is due to take part in a mass march and make a speech later in Madrid.
At the last U.N. climate event in September, she harangued delegates, demanding: “How dare you?” and declaring, “You have stolen my dreams.”
By this meeting’s close on Dec. 13, negotiators hope to resolve remaining disagreements on how to implement an agreement struck in Paris in 2015 to limit a rise in global temperatures to between 1.5 and 2 degrees Celsius over pre-industrial levels.
They saved the deal from disintegrating last year by agreeing guidelines for putting it into effect, but current pledges fall well short of the temperature target and sticking points remain among signatories.
President Donald Trump has started the process of withdrawing the United States, the top historic greenhouse gas emitter, from the accord.
Spain has said a desire to prevent this move from sapping morale around the project was one of its reasons for offering to host after riots over inequality prompted Chile to withdraw.
The Chilean delegation remains responsible for presiding over the negotiations. (Reporting by Isla Binnie, Sebastian Rocandio, Marco Trujillo and Jessica Jones; Editing by Alison Williams)