The Government wants to security scan at least a quarter of all train passengers for explosives, knives and guns to protect railways and the London Underground from terrorists.
The Home Office has published details of what it wants the scanners to detect and how they should work, and is asking for advice on the technology available.
"The main focus is on the detection of explosives and weapons on people and in bags," the research brief states, suggesting that technologies including X-ray, magnetometry, vapour and trace methods, electromagnetic radiation and ultrasound could be used.
The researchers also want to know whether wheelchairs, false limbs, crutches, pushchairs, and bikes could be scanned and whether so-called dirty bombs could be spotted.
Crucially, the document insists the scanning must be done without holding anyone up.
"Any screening methodologies proposed must not delay the passengers any more than they are currently as they pass through the station," it states.
The scale of the challenge is highlighted by official figures which show almost four times as many rail passengers arrive in central London in the morning rush-hour period as pass through the security scanners at Heathrow and Gatwick airports all day.
The Department for Transport, which is also involved in the project, said it wants to balance "the need to protect passengers with the need to travel freely and easily on rail networks."
Aware of the limitations of existing scanning methods, the Home Office is asking for future technology to be assessed in addition to commercially available equipment.
It wants to know how many people might be able to go through each scanner when one in four is being actively scanned, and what the rates would be for hazard detection and false alarms.
But a leading detection company told Sky News the Government's wish-list is unlikely to be achieved in the foreseeable future.
Kromek , in County Durham, designs and builds cutting edge scanners including one that can that can differentiate between water and paint thinner held in a metal container.
Commercial director Nigel Day said even with predicted screening technology advances, the quickest security check inevitably involves some kind of delay.
"There would be too many people trying to move too quickly through a security checkpoint with various different items," he explained.
And he predicted that airport screening, which is the main focus of scanning technology development, is unlikely to be transferrable to the rail system.
"We've already seen the challenges in aviation security," he pointed out. "They're only going to be magnified in rail transport."