http://www.cnn.com/2005/US/08/03/transit.security.ap/index.html
My opinion?
Random checks are statistically stupid.
To get an accurate sampling of passengers you would have to search the majority considering how few folks there really are acting as terrorists.
The answer - same as for the airlines.
Search everyone upon entry.
For the subways a fella can enter; refuse search and leave and enter into another entrance and likely get through.
The fact that we have to go through this is just crazy and it's another layer of freedom terrorizm has taken from us. We now cannot move safely within our own cities. Next question will be "Papers please?"