At least with handheld scanners they can have a more random or better, targeted (inteligance led) approach, and the costs will be much lower than with fixed scanners. What concerns me though is that metal detectors (which I presume they are) will give a lot of positive readings on people using trains (at least with airport scanners, you know that many metal objects are banned as hand luggage, so are less likely to set the detector off), an if people are then searched, it could cause trouble if they then miss their train and it turns out they havn't got anything illigal on them. At least with airports, you know to leave plenty of time to get through serurity, and they would probably wait anyway. Also, you'd need a police officer to search people, and this ties them up from other duties, and if a number of people set of the detectors you could have a que forming. Again though, it's inteligance led, so when scanners are use their is a threat, people will be fairly happy, but if it's targeted at particular stations, or just random, people will probably soon become annoyed. Also, targeting particular stations will just divert people wanting to get past the scanners to other stations, and random scanning is putting it down to luck (imagine if they had been scanning at other stations in London, but not Kings Cross Thameslink on the morning of the 7th of July, or if they had regular checks there, but the bombers knew and got on a MML train to St Pancras instead).
