Passengers at Union Station were subjected to a search of their bags Wednesday afternoon before boarding Metrolink trains.
Sheriff's deputies and Transportation Security Administration officers were conducting the searches.
Metrolink received a federal grant earlier this year to do the searches and left fliers for passengers telling them that they would begin soon.
At the time, agency officials said that the searches were not in response to any specific threats, but instead were being done as an overall push to tighten security, as other rail carriers have done.
The agency has been searching passengers' bags at other stations, but Metrolink spokesman Francisco Oaxaca said this was the first time that searches were done at Union Station, the hub of Metrolink's rail network.