Climate activist Greta Thunberg has tweeted a photo of herself sitting on the floor of a German train surrounded by lots of bags.
It came as marathon international climate talks in Spain have ended with negotiators postponing a key decision on global carbon markets.
She posted the tweet late Saturday with the comment: "Traveling on overcrowded trains through Germany. And I'm finally on my way home!"
Some Twitter users pitied the 16-year-old Swedish activist for not being able to get a proper seat on the train for the long ride home from Madrid, where she was attending the UN climate change conference.
They wished her a safe trip home after months of travelling by trains and boats to different climate events in Europe and the US.
The activist does not fly on planes because it is considered harmful to the climate.
She was recently named Time magazine's Person of the Year for her efforts to prod government and others to take faster actions in fighting climate change.
German railway operator Deutsche Bahn has come under fire in recent years for delays, last-minute train cancellations and expensive ticket fares.
Deutsche Bahn replied to the teenager's tweet, wishing her a good trip back home, adding "we continue working hard on getting more trains, connections and seats."
In the picture on Twitter, Greta is sitting on the floor at the end of a rail car with her back leaning against a suitcase, staring out of a window.
There's an empty food box next to her and more suitcases and backpacks piled up by her side.
After two weeks of negotiations on tackling global warming in Madrid, delegates from almost 200 nations passed declarations calling for greater ambition in cutting planet-heating greenhouse gases and in helping poor countries suffering the effects of climate change.
But despite holding the longest climate talks ever in 25 nearly annual editions, they left one of the thorniest issues for the next summit in Glasgow, in a year's time.
Environmental groups and activists accused the world's richer countries of showing little commitment to seriously tackling climate change.
