McCollum's criminal history is a quirky, precocious one, turned sad. As a boy growing up in Jamaica, Queens, (a large suburb with a promising mess of train and subway stations, bus depots and railyards), he exhibited a strong fixation with New York mass transit. He would miss school to befriend subway drivers and conductors, who in turn taught him to drive trains and buses and gave him his first MTA uniform when he was a teenager.

His first arrest was sensational. On a winter's night in 1981, Darius was riding with the driver of an E train when the driver became ill and asked him to take over. The driver got out at 34th Street and Darius drove downtown to the World Trade Center, his passengers unaware, until it was time to turn around. He was apprehended as he walked back through the train to start the journey back uptown. He was only 15.

Since then, and with dismal repetition over two decades, Darius McCollum has been arrested 20 times for a catalogue of minor crimes: impersonating MTA officials, exploring subway tunnels in the Bronx, signing out 13 buses out for drives around the city and then bringing them back. In 2000, he tripped a subway car's emergency brake and administered a perfect evacuation of the train. After some of his spells in jail, McCollum's parole conditions have demanded that he remain in New York City, where he keeps getting into trouble. Newspaper articles about him, which he carries with him most of the time, have called him a "bus rustler," a "transit kook" and a "train in the neck".

McCollum's parents have always known it wasn't just for kicks. Even when he was a young boy, they asked for psychiatric care for their son. For a profile in Harper's magazine in 2002, they handed over some of the notes Darius made as a boy to describe his adventures on the subways, which could last for days at a time. The log for the days of his first arrest read as follows: "late for school-in at 10:00 am; home; out-drove train; court."

McCollum's life of subways and prisons has straddled the emergence of Asperger Syndrome, also known as "Little Professor Syndrome", into serious medical study. Asperger Syndrome (AS) is used to describe people at the higher end of the autism spectrum and was only accepted into the American medical canon in 1994.

A genetic condition, AS is usually noticed in children who show a majority of as many as 12 symptoms, which include some measure of difficulty in social interaction, strange body language, a very literal understanding of the world and a tell-tale fascination with what is frequently a mechanical subject. Medical studies record people with AS whose lives are consumed by electricity pylons, burglar alarms, vacuum cleaners, washing machines, video players, calculators, computers, planes, and clocks.

Since the late 1990s or so, medical experts and people in the AS community recognized McCollum as someone who might be "on the spectrum", as they say. After a judge rejected an Asperger defence in 2000 for another of McCollum's crimes, his lawyer, Stephen Jackson, finally had him formally diagnosed. And when he was released from prison in early 2004, he started attending classes run by the Global and Regional Asperger Syndrome Partnership (GRASP), a support and advocacy group in New York.

"When you meet Darius McCollum, one of the things you immediately realize is how likeable this guy is," says Michael John Carley, who runs GRASP and who was himself diagnosed with AS in 2000.

According to Carley, a passion for train systems is a "leitmotif" of Asperger's. Not only are they a good source of data to memorize and routes to understand, but they are also outside the world of subtle, social decision-making that he says people with AS find most frustrating.

"There are a lot of train-loving people among our folk," says Carley. "Trains move forward. And then there are the tracks! You just sit there. You don't have to decide which way to go - and that's an incredible relief to all of us."

Despite an apparent willingness to be there, McCollum only managed to get to three support classes before he strayed once more into the yards and tracks that fascinate him the most. On June 11 last year he walked into the Long Island Railroad yards in Queens and started asking about the new M7 locomotive that was standing there. When asked for identification he started to leave, and when he was detained he was found to have stolen a set of keys to the train.

McCollum stayed in custody until his trial in February. Although his medical condition seems to be clear, his lawyer Stephen Jackson, "after extensive negotiation," advised him to plead guilty to the charges. Jackson explained that McCollum's record is such that if he had taken the case to trial, there was a possibility that he would face the rest of his life in jail. As it is, he should be out by Christmas.

But what next? This week the New York Times suggested that the MTA should think about finding room for McCollum, comparing him to Frank W. Abagnale Jr, the basis for the film, Catch Me If You Can. Jackson, on the other hand, remains adamant that McCollum should leave the city, and head for North Carolina, where his parents have retired. "I am optimistic… Darius himself wants to get better," he says.

Michael Carley is more sanguine about McCollum's future. He too wants him "the heck out of here… away from any place that has a subway system or any commuter rail system." But he fears that McCollum's life of intermittent freedom and imprisonment, subway tracks and prison yards, may have bred a kind of fatalism in him. Together with his minor celebrity, which he enjoys, McCollum is in a cycle which will be hard to break.

"What I'm worried about is that he is used to this: he's used to being in jail, this is his life," says Carley, who still talks to McCollum regularly on the phone. "I'm sure part of him just thinks he is a criminal, that that's what he is."