NJ Transit says the search is in the Elizabeth area, just south of Newark.
Agency spokeswoman Penny Bassett Hackett says the activity is related to a possible suspect in Saturday's killing.
At least 11 NJ Transit trains have been stopped for up to an hour on the Northeast Corridor and North Jersey Coast lines.
Police are seeking Eric Floyd on a murder charge in the death of Sgt. Stephen Liczbinski (LIZ'-bin-ski), who was shot while responding to a bank robbery.
Another suspect is in custody in Philadelphia; a third man was fatally shot by police.
The number to give information to help police in the investigation is (215) 686-TIPS [ (215) 686-8477 ].
