Shea said Exclusive Advertising reacted immediately. 'Every time we had an incident like that, we took a car out of service and they reprogrammed the message board by deleting the messages and programming in again the correct messages,' he said. 'Now there's a fix for it that, to my knowledge, was not available back six years ago. And it's having the programming device password protected.'

Shea said Exclusive Advertising is in the process of password protecting message boards in all the trains.

Although Shea didn't know exactly how the hacker managed to reprogram the message boards, Greg Donohue, president of Exclusive Advertising, offered some insight.

'We have about 800 of these LED scrolling message signs throughout the fleet of trains on the GO system,' Donohue said. 'The signs are programmed via an infrared remote control. When we bought the signs about six years ago, it was relatively new technology -- and at the time the signs weren't password protected.'

Donohue said that while anyone with that particular remote control could reprogram the signs back then, it the remote control devices weren't available publicly. 'You had to buy them through a distributor or the manufacturer and they were sold specifically for industrial use,' he said. 'But what's transpired over the years is that they are becoming available to the public through retail stores, i.e., Sam's Club. And you can buy the signs bundled with the remotes through other retail outlets. Because they're all on the same frequency, anybody that buys the sign and has the remote can reprogram signs. So what you need is to password protect each one. That's what we're doing now, password protecting them. So then you need a password to alter the message on the LED screen.'