A day after city officials warned of a possible, imminent attack on the underground train system, there was no noticeable reduction in commuter volume on Friday although some passengers confessed to being more nervous than usual.
  
The sense of unease was heightened by an apparent hoax at Penn Station, one of Manhattan's main transport hubs, part of which was cordoned off as police in bio-hazard suits, National Guard troops and sniffer dogs patrolled the area.
  
The scare was prompted by the discovery of a suspicious package that turned out to be trash, and a bottle containing what Police Commissioner Ray Kelly described as containing dye and acid. 
   

New York city officials have 
defended the terror alert  

"It looks like a prank," Kelly said. 

Despite the heavy security response, no order was given to evacuate the station, and disruption to services was minimal.
 
"There was no danger to anyone at anytime," an Amtrak train spokesman told reporters.
  
Security on the city's transport network was ramped up on Thursday evening after Bloomberg said federal agencies had passed on a highly specific threat of a "terrorist" attack on the subway "in the coming days".

Non-credible threat
  
Officials refused to detail the precise nature of the threat, although police underground train patrols intensified random searches of backpacks and baby strollers which had been cited as possible means of concealing explosive devices. 

New York's train network carries
4.5 million passengers daily 

Federal officials played down the nature of the threat, which Department of Homeland Security spokesmen reportedly dismissed as "non-credible".

A Pentagon spokesman said the US military gathered intelligence related to the New York threat in the course of its operations overseas.
  
The New York underground train network, fourth largest in the world in passenger volume, carries 4.5 million passengers on an average working day.
  
The city's Metropolitan Transportation Authority announced in August that it had awarded a $212-million contract to US defence giant Lockheed Martin to build a state of the art security system throughout the train and bus network.