Brown said 250 rail stations would be retrofitted with anti-terror devices by 2011 in a £3.5bn annual package.
However, while buildings can be modified relatively easily, the demands of mass-transit transport make it difficult to introduce without causing significant delays to passengers' journeys.
"Scanners on the train or tube would be impossible," said MFD Security director Chris Bowes, a civil engineer whose firm advises the Ministry of Defence on security matters.
"There would be horrendous difficulties."
Bowes says that a great deal of work has already been donein traffic management and introducing furniature or bollards to the street to keep car bombs at bay.
While this is effective for buildings, Bowes said it would fail to guarantee the safety of members of the public.
The Department for Transport (DfT) has been conducting tests with portable scanners and sniffer dogs with Transport for London to assess the impact of searching on passengers.
"This is not about permanent screening at every station," said a DfT spokesman.
"It would be unworkable to require 100% passenger screening of the sort you see at airports- the delays would be unacceptable to
passengers and the costs very high.
"We are aiming at a strategy that translates into deterrence, detection and public reassurance but is proportionate to the threat and minimises disruption."