Securing our nation's mass transit systems seems like a nearly impossible task and there's certainly no silver bullet for protecting the traveling public. A new report issued by the Government Accountability Office in July found that there are a number of promising explosives detection technologies out there, but also noted there are serious limitations that need to be addressed for proper deployment in a rail environment.
The report found that handheld, desktop, and kit-based trace detection systems, x-ray imaging systems, as well as the use of canines, are all technologies that have demonstrated good detection capabilities, but did not recommend any of these technologies specifically.
One of the concerns in securing ground transportation is passenger flow. It's fairly understood that passengers on New York's subway system, for example, are not going to tolerate major interruptions of their commute. Therefore, the government needs to find technologies that can detect explosives, but do not impede on passenger flow. For this, the GAO recommends the development of a concept of operations that "would help balance security with the need to maintain the efficient and free flowing movement of people. A concept of operations could include a response plan for how rail employees should react to an alarm when a particular technology detects an explosive."
The GAO also reported that in implementing these technologies and policies there are possibly too many organizations involved in this effort:
While there is a shared responsibility for securing the passenger rail environment, the federal government, including TSA, and passenger rail operators have differing roles, which could complicate decisions to fund and implement explosives detection technologies. For example, TSA provides guidance and some funding for passenger rail security, but rail operators themselves provide day-to-day-security of their systems.
TSA seems to be taking a bigger role in securing surface transportation. Secretary Napolitano recently announced the agency (and its new head) will focus more of its efforts on securing mass transit. It recently launched a national "See Something, Say Something" campaign, but no specifics on the technology side.