My conclusion about what to to about it is that vital services need their own emergency generators, preparations for icelanding random parts of the grid is a good thing and the same is true for emergency stockpiles of spare parts and emergency repair groups. Doing something about the possible terrorists is much harder.

My conclusion about why no such terrorist attack has been done is that intelligent terrorists look for attacks that have a large symbolic value and not attacks whose sum of small inconveniences and costs add up to very large sums. They are attacking symbols, not the basic and resilant economy.

The grid risk is locally handled in a few ways, none of them wholly satisfactory for a defence nerd.
The main grid gets a fairly small increases in redundance to handle increased trading on the nordic electricity market and lowering the risk for accidents bringing down part of the grid. The investments in control systems are probably more significant and as far as I know there are plans for local icelanding in case of a grid break down. How it works and how tested the present systems are is probably confidential.

There is an emergency repair organization who has everything needed for rebuilding small sections of 130-400 kV lines or building a number of emergency switchyards. I do as a defence nerd find it to small a force but it has been planned to be air mobile with common military transport aeroplanes and use small helicopters for movement of parts while working in case it will be needed internationally. If terrorists blow up your grid request aid from Sweden, probably good foreign policy planning, I hope we then ask for some future favor in return. It was used about 1.5 years ago when the storm/weak hurricane Gudrun to peoples surprise brought down 130 kV lines, the largest size that use pairs of wooden poles with complete tree fall clearance. Tree fall clearance, not fully grown trees flying around clearance.

It should be such that the telephone infrastructure, all police stations, fire brigade stations, hospitals, (disabled)old peoples homes, water and sweage works etc should have their own emergency generators. But it is not complete, the investments seems to increase every time there is an accident. Its seems like the trend is to mostly increase the number of installations to recieve mobile emergency generators (Essentially a box with a big switch, a connector, fuses, grounding and control light to indicate that mains power is back. ) and when there is some major problem it is discovered that fallen trees are in the way of the gensets, or thet they are too few, hard to organize and so on and the number of fixed installations goes up in the voulnerable spots.

The old civil defence goal for the water supply were that every area supplied by a municipiality should have full redundancy in case one water source were poisoned or one water works broke down. This goal were not reached but a fair number of towns have full redundance. But I think my own home town is growing out of it and would ration garden watering. The planning part is still maintained, there are lakes and streames tagged as possible water sources, some natural sand formations from the ice age are set aside for water cleaning and so on. But I think stockpiles of pumps and tubing whent to the scrap yard with the big drawn down after the cold war. But such redundancy is cheap in Sweden since most of the country has nearly unlimited fresh water from lakes and streams. It usually only require some sand filtration or the common artificial ground water treatment by using a natural sand formation as a giant sand filter. Most or all larger water works chlorinate, mostly to keep the water from loosing drinking quality in the water lines.

This has of course not stopped the crazy trend of bottled drinking water even when average bottled water has lower quality then random tap water. Good marketing can get people to buy and carry home a liter of water for one dollar instead of drinkig water from their tap for 0.1 cent. I had a good laugh at an evening tabloid this weekend that tested different brands of bottled water without added taste exept minerals, tap water won. This bottled water trend will probably be a quick road kill for PO.

With that little HE we would be talking about 1 pumping stn, or a smallish breach in a single pipeline yes?

"Drinking water you could bring in by sea or train or trucks"

I would imagine you could repair the breech faster than you could organize the mass distribution of alternative supplies. It would take some number of days for crops to die, and most households have a 40 gallon emergency supply of potable water on hand at all times (in their water heaters) though I suspect most have never thought of it this way, so the actual death toll I would expect to be minimal.

Now on the other hand dropping hydro towers could do a lot more damage I would think, and use the HE to greater effect since the amount required to sheer 2 legs on a tower is quite modest if rigged correctly.

The east coast blackout of a few years ago which originated from a single sub-station failure at a time of max demand gives a good example of the cascade effect on a deeply coupled system close to overload at the best of times.

A distributed attack focusing on the major interconnecting lines between cities, and the interstate tie lines, all of which are well documented in open source files. Hmmm...
I hear my file starting to be added to, time to stop now ;)

One thing that has puzzled me for years is why terrorists have not yet taken out the pipelines that supply water to So. Calif.

I'm sure there are more flammable targets ;-)

Beyond that, I think the system is more redundant that casual observers would note (particularly there are a number of near-in reservoirs and lakes holding short-term supply).

While I'm doing notes - one amusing thing is that while we've got those big pipes going over mountains ... many of them have traditionally been siphons.  While the water goes a long ways, it is "net-net" downhill to LA.  Again details on total system electrical consumption in the WATER-ENERGY RELATIONSHIP by the California Energy Commission)

Same way with transmission lines, fixes are fast and internal supplies to back up outages plentiful in basin.

Even the amount of local refineries in LA are enough to take care of all its needs.