This is nefarious-sounding: The Department of Homeland Security "spent millions of dollars on mobile body scanner technology that could be used at railways, stadiums and elsewhere," according to a trove of documents obtained via FOIA by the Electronic Privacy Information Center.
The documents, beginning with one titled "Rail Security Pilot Study Phase II at PATH", go back a few years and detail proposed studies and contracts with scanning technology manufacturers, discussing the potential for security measures such as body scanners in public places like mass transit. Phase I of the Rail Security Pilot, for instance, was oriented around evaluating technologies that don't "require the collection of personally identifiable information" like x-rays. But Phase II, discussed in the documents, talks about evaluating technologies "with a potential privacy impact" like whole-body infrared images, whole-body millimeter wave and whole body terahertz images. The study proposal also talks about how passenger privacy would be protected during the study (by anonymising the data collected, blurring faces and the like).
The crazier stuff in the docs is what's referred to as "intelligent video", using "multiple static cameras at different corners of a Z Backscatter Van"-think a giant, body-scanning van-along with cameras mounted on poles and buildings, designed to detect and track "one or more persons and objects using multiple cameras". In other words, the DHS had Northeastern University working on technology to make it possible to scan and digital strip-search people basically anywhere and everywhere, like walking on the street.
The overwhelming concern in these documents is thwarting suicide bombers and "leave behind bombs." A really fascinating detail though: One of the TSA's key performance parameter for a "standalone backscatter x-ray system for imaging of suicide bomber explosive devices" to be developed by Rapiscan - i.e. like one of the backscatter body scanners at the airport-only require a 90 per cent success rate in detecting concealed explosive devices. For all of the hassle and grandstanding and security theatre, it is within the TSA's acceptable parameters that if 10 terrorists attempt to smuggle an explosive device through one of these scanners, one of them gets through.
All in all, the documents are pretty mind-blowing: Basically all of the worst things you've considered the DHS and TSA might want to do, they've at least thought very hard about doing it, even if, as they told Forbes, "TSA has not tested the advanced imaging technology that is currently used at airports in mass transit environments and does not have plans to do so." It doesn't mean they won't.