Great idea, then all the terrorist have to do to bring this country to a halt economically is knock apart a few rail tracks. Same for an invading army, but lets face it, that's less likely. Trucks can be rerouted to any number of roads, rail cannot.
During WWII, the British rail network took a very heavy pounding from the German bombers. And yet, somehow it managed to continue providing the service of transporting goods around the country.
The reason was that it had excellent built-in redundancy. Multiple available routes between all the major destinations made it remarkably robust. Sure, some routes are quicker than others, but the ability to keep things moving even if two or three main lines were knocked out was critical.
So history shows your argument to be incorrect. Rail can be just as robust as roads when it comes to network reliability.
And in fact, your counter point that trucks can be routed onto any number of roads is also incorrect, as heavy vehicles can only go on roads that are suitably robust. You may look at the map and see a whole network of roads, but filter it down to the routes that could be used by significant number of trucks, and you'll see that in fact it wouldn't take nearly as many road blockages as you might think to severely affect the country's ability to transport goods.
Sadly the British rail cuts in the 1960s removed a lot of the "unnecessary" lines, which left the UK today with a much more efficient network, but one which would not survive a similar bombardment now.
And this points to the final part of the discussion: maintenance costs. Both rail and road networks require significant maintenance. Many will point to the roads as being cheaper in this respect, but in fact heavy trucks cause a lot of road damage. The main reason roads appear cheaper is because the cost is not direct, but the two are comparable. Subsidies and taxes also mask the real costs.