BOSTON (AP) — A Boston commuter rail conductor is being commended for helping uncover a multi-million dollar scheme to create and sell counterfeit MBTA passes.

MBTA Transit Police Chief Paul MacMillan will present the conductor with a commendation Wednesday. The action of the employee helped uncover the largest case of fare fraud in the MBTA’s history.

In March, the conductor was collecting tickets from commuter rail passengers when he noticed a color discrepancy on a monthly fare pass. He confiscated it and reported it to his supervisors, who immediately notified MBTA transit police.

An investigation led to the indictments of four people.

Prosecutors said one man made the counterfeit passes while working at a company contracted by the MBTA to make the passes, which were allegedly then sold on Craigslist.

Copyright 2011 The Associated Press.