What was the "bomb" that held up trains in Acton yesterday?

While T police were busy wrestling deadly asthma inhalers to the ground in Woburn, something at South Acton held up four trains. Gary McGath, who takes that line, wonders what it was and why there wasn't anything in the papers about it:

... Probably the MBTA is too embarrassed to admit what the "suspicious package" turned out to be.