THE CityRail network has insufficient electric power to run the biggest order of new trains in Australia and has been forced to spend more than $1 billion to fix the problem and to pay for other essential upgrades.

The first of more than 600 rail carriages due to begin testing in October will be restricted to one section of the rail network until a program of capital improvements is completed, some of which are running years late and over budget.

Although RailCorp was aware more power was needed for the trains, the budget for the overhaul has blown out almost seven-fold.

If too many of the trains are put into circulation at once, sections of the network will short-circuit, sources have confirmed.

In a tender last year the State Government revealed "RailCorp does not currently have an enterprise-wide change strategy for achieving business readiness to accept [public-private partnership] implementation, and this represents a major risk".