The subway is full of subtly beautiful things: light slipping up the tracks as a train approaches, the three notes a train’s electric motor hits as it revs to full speed, the leaky, noir atmosphere of an abandoned station at night. None of these, though, is as refreshing as the small poems posted in cars’ advertising panels.

The poems, part of a 14-year joint venture between the Metropolitan Transportation Authority and the Poetry Society of America, are seen in only 40% of the transit system, so they aren’t something a straphanger comes to expect. The words can appear as a pleasant surprise to those entering a car, and New Yorkers have different reasons for allowing their eyes to settle on them.

“I just read ’em,” an operational engineer who lives on the Upper West Side, Renard Herbert, 44, said. “I’m not too into writings and stuff like that. I basically glance so I don’t have to make eye contact with people on the train.”

Like puzzles, the poems can be unraveled or simply read for relaxation, riders said.

“Sometimes, they’re a little bit mind boggling, but it’s poetry. It’s supposed to be mind boggling,” a database administrator from Boerum Hill, Brooklyn, April Salazar, said as she leafed through the latest New Yorker. “It will make you sit and think a bit.”

The task of making the final decision about which poems will be posted to be read by millions of riders is up to the MTA’s vice president for marketing, Alicia Martinez. The rules are simple: The poem can be no longer than 12 lines and each line cannot be much longer than 10 words. Excerpts are used only rarely, and only when they can stand alone. Poems are never edited — the author’s words appear as they were written.

“It’s for New Yorkers,” she said. “But the fact is the New York riding public is so diverse. I try not to choose simple poems, but intelligible poems. You don’t have time while riding the subway to read and reread and analyze the poem. We all try to find poetry that is really good, really important, and also accessible.”

Recent topics range from reminisces about childhood (“Those Winter Days,” by Robert Hayden), to the place of nature in a frenetic world (“A Leaf,” Bronislaw Maj). The poems come from all eras — from the 13th century, “Out Beyond Ideas of Rightdoing and Wrongdoing,” by Jeladdin Rumi, to the 20th century, “The Blue Boat,” by Kathleen Jamie.

Possible selections are shuttled back and forth between the Poetry Society and Ms. Martinez’s office, and eventually the eight to 10 poems that will adorn the subways for the year are chosen and printed. Ten other cities now have their own “Poetry in Motion” programs, but those with an eye for subtlety will notice that every city has its own style and tone, Anita Naegli, a writer and the Poetry Society’s director of the program, said. Some smaller towns focus on local talent, and the Fresno program prefers Californian poets. Classics comprise a large portion of New York’s works, a change Ms. Martinez implemented when she took over the program six years ago.

The idea of putting poems in subways first came from London, a point Ms. Martinez regrets, considering the program’s popularity in New York. Still, she isn’t surprised, considering what she calls the impressive sense of language in that city’s subway system.

“It’s so wonderful how literate they are,” she said. “Even the ads in the system have more copy than we would ever dare to write.”

A poem that ends up in New York’s subways could come from anywhere. There is no formal method for finding the right one — it’s a matter of tone, length, and variety, and the poem should somehow speak to the hearts and minds of New Yorkers.

“A lot of it comes from remembering, going through anthologies, or picking up a volume of Yeats because you really love Yeats,” Ms. Martinez, who has a doctorate in literature from Columbia University, said. Her dissertation was on Percy Blythe Shelley’s “Revolt of Islam.”

A pressing concern is to avoid consistently posting poems with dreary tones, as was the case with the last two year’s Shakespeare selections: the “Today” speech from Act V, Scene V, of “Macbeth” and Iago’s speech in Act III, Scene III. The subways, the thinking goes, are dark enough.

“I like it ’cause it rhymes sometimes, but it still makes sense, and it gives you something to read, something nice to read,” an office cleaner from the Bronx, Essence Smalls, said. “It just makes me smile when I read it.”