This is worth a thought. Make the commuters more viligant towards unclaimed baggages. Travellers should keep a watch on all the baggages that go on top rack as well as those underneath the seats and between gangways. With the number of people crammed in the train, an intuitive decision can be very easily made about what areas to cover, so that nothing is lef to chance.
CCTV have found to be useful in finding the culprits, but they don't generally help you STOP a crime. CCTV monitoring is not effective, because of the monotony of work - you necessarily need a person to stare endlessly into the many monitors to decide if there are any suspicious elements within the milling crowds. And how do you delve in their baggages from the TV. It's just not effective way to tackle crime and terrorism. Probably, it can be used to monitor restricted areas - say railway yards, train under-sides - where it may be possible to stick in the bombs. It would be easy to detect, because of the surreptitious nature of the activity.
Meanwhile for bombs in parcels, commuter vigilance only seems simple and effective solution