A deal worth £50 million has been struck to upgrade the power supply of Sussex's railways.

The announcement means new rolling stock can be introduced to replace the out-dated slam-door trains on the Brighton-to-London line and beyond.

The new trains have heavier carriages and require more power to operate automatic doors and air conditioning.

Network Rail, the not-for-profit company that replaced the troubled Railtrack, confirmed deals had been signed to upgrade the supply in Sussex, Wessex, Kent and inner London.

A spokesman said: "Network Rail, working closely with the Strategic Rail Authority and the train operators, has spent many months identifying the overall size and scale of the power supply project.

"More than 500 people will soon be working on the delivery stage of the power supply upgrade and with initial substation and cable procurement already well advanced, Network Rail anticipates substantial physical works to start on the ground by late spring."

The Sussex scheme was awarded to a joint venture between Integrated Utility Services and Peterhouse.

Peterhouse chairman David Jackson said the deal was worth £50 million in the first year.

He said: "Some people see rail as risky. We see it as a massive opportunity.

"Rail is of major strategic importance to Peterhouse. Winning this large and prestigious contract for the upgrade of power supplies demonstrates our position as a leading contractor within the sector."

Each of the four contract winners will work with Network Rail to design, construct and install the new equipment in its area.