- You may be looking for the Torchwood story End of the Road.
The Doctor lands on what appears to be a subway platform. Soon, he finds himself chased by zombie-like creatures. As the creatures appear to gain the upper hand, a masked figure suddenly rushes the zombies. They are quickly dispatched, and the Doctor's shadowy saviour removes her mask. She introduces herself as "Angel", one of the Guardian Angels who patrol the under-city. She explains that the zombies were in fact called "Cannibals", in deference to their eating habits.
Angel suggests that the Doctor should come with her back to Guardian Angel HQ. Once there, she introduces the Doctor to her fellow Guardian Angel, Sonny, and their scientific advisor, the Engineer. The goal of the Angels, it turns out, is to try to do something to reverse or escape the crushing levels of pollution in the city, as this has been traced directly to the creation of the Cannibal underclass. The Angels, in fact, believe that there is some place beyond the city, somewhere at "the end of the line", called "countryside". If they can just figure out how to manipulate the subway system, perhaps they can break free and find it.
The Doctor offers to help, just as Angel HQ is rushed by Cannibals. The Doctor believes that he's managed to set the tracks properly to lead the Angels out of the city, but he gets separated from his new friends after he believes he sees them board the right train. Thinking all is probably well, he slips away from the Cannibals and finds his way back to the TARDIS.
Just as he's about to leave the planet, though, he reasons that he didn't actually see Angel and Sonny and the Engineer make it to their new life in the countryside. So he materialises the TARDIS to the end of the subway line. There, he finds it's not a paradise, but really just as polluted as the city proper. Nevertheless, he waits for his new friends to arrive. After a considerable length of time, a radioactive rain storm phases the Doctor's determination to wait for his friends to arrive. He goes back to the TARDIS, saddened that his friends didn't make it, but also grateful that their unrealistically hopeful dreams of the countryside were likely still intact.
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- In part 1, the Fourth Doctor bolsters his own confidence in a bad situation by quoting the Third Doctor's dying sentiment, "Where there's life there's hope." (TV: Planet of the Spiders)