Here's a short story:

An arm, legs, underwear, dark jeans and size 5½ Air Jordan sneakers turned up on the Queens side of the East River. The jeans and shoes were the same size of Avonte Oquendo, an autistic 14-year-old last seen last fall walking unsupervised out of his school. 

Yet his mother was steadfast: "It's not Avonte until it's Avonte." 

That time has come. 

On Tuesday -- five days after those body parts and scraps of clothing were found -- Vanessa Fontaine learned that DNA tests proved her son's remains indeed had been found. 

Her lawyer said that Fontaine has been stoic, strong, focused and hopeful throughout this ordeal. But after police told her the news on Tuesday, "she finally just broke down ... just crying and crying." 

"I kept saying, Vanessa, say it again," David Perecman said of his muddled phone conversation. To which she replied through the tears, "It's Avonte, it's Avonte. (The police) came. It's Avonte." 

So ended not just her family's search for the teenager, but that of the United States' most populated city. Police deployed sniffer dogs, combed surveillance footage and repeatedly canvassed each of New York City's 468 subway stations because of Avonte's love of trains. 

Missing teen's fascination with trains shifts search 

The most poignant, most personal part of the search was Fontaine's recorded voice that was broadcast from patrol cars and other search vehicles. Avonte couldn't communicate verbally and had the mental capacity of a 7- or 8-year-old. But Fontaine hoped that he would hear her and head toward a police car's flashing lights to safety.  (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: when did Fontaine find out the news?
Guess: On Tuesday

Question: was she sad?
Guess: yes

Question: who did police find?
Guess: Her sons bdy

Question: how old was he?
Guess: 14

Question: what was his name?
Guess: Avonte Oquendo,

Question: was he autistic?
Guess: yes

Question: what was his mental capacity in years?
Guess: 7-8 yr old

Question: what recording did police use while looking for him?
Guess: Fontaine's recorded voice

Question: what size sneakers did Avonte wear?
Guess: 5 1/2

Question: what kind of shoes?
Guess: Air jordans

Question: where did they find those?
Guess: Int he east river

Question: what proved it was her son?
Guess: DNA

Question: who kept saying "say it again"?
Guess: David Perecman

Question: did police try sniffer dogs?
Guess: Yes

Question: how many subway stations did they search?
Guess: 468

Question: what else did they comb through?
Guess: Footage

Now answer the following question in the same format.

Question: why did they focus on subway stations?

His fascination with trains