It may be cheap to monitor a single spot. It is extremely expensive to monitor everything.

There is a balance like always. It seems odd that they have zero cameras on bridges and other main infrastructure, although I believe that level crossings tend to have them (perhaps more to avoid liability in case of accidents, though...)

Main point is that there aren't technical difficulties in verifying the state of main infrastructure in real time (contrary to the claim of the commenter I was initially replying to), and it's more a question of priority and will than doability or cost.

It will happen but the usual way is that "it's not possible", "it's too expensive", etc until something bad enough happens and then suddenly it is doable and done.