Land and Mass Transportation Security

BOSTON — The federal government has rejected a request by the MBTA and other big-city transit agencies to use antiterrorism grants to install systems that detect chemical threats in subway stations.

The MBTA has had the technology in one of its stations since 2001, as part of a pilot project, and acting MBTA Police Chief Paul S. MacMillan wants to use an estimated $3 million from this year's Homeland Security grant to outfit three more stations.