The ring has been investigated for the past two years.
It's unclear how much they stole, but it is believed to be at least $200,000.
The head of the union says the suspects were taken off the job earlier this week.
Eight of the defendants pleaded not guilty on Friday morning. The remainder will be arraigned later in the day.
Charges in the indictment unsealed against the first eight vary from conspiracy to grand larceny to possession of stolen property. Attorneys for several of the defendants said the copper wiring allegedly stolen was scrap that was to be discarded.
"I'm shocked, and I'm embarrassed," said Christoper Natale, of the Brotherhood of Railroad Signalmen. "This is not indicative of Long Island Rail Road employees. We would never condone this type of behavior. If they did something wrong, they're going to have to pay for that."
Natale says he's also waiting to get more information on how exactly the 15 workers allegedly stole the copper wires, most likely from local rail yards.
He says either way, it's not a good thing for the LIRR or the union.
"We're not criminals," he said. "We're just guys who have families who want to go to work. When something like this happens, it gives us all a black eye, and I feel bad for the MTA, too. No one needed this."
Copper sells for about $3.70 a pound.
The MTA had no comment on the investigation.
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