UK to unveil plans to electrify two rail lines

* Investment aims to speed up journeys, cut emissions

* PM Gordon Brown says will announce details on Thursday

LONDON, July 22 (Reuters) - Britain plans to electrify two railway lines in a move designed to reduce carbon dioxide emissions and speed up journeys, Prime Minister Gordon Brown said on Wednesday. "Tomorrow I will announce a major investment in train electrification on two important lines, investment that will secure the modernisation of much of our railway infrastructure, creating and protecting hundreds of jobs," Brown told reporters.

He gave no further details, but media reports said the line between London and south Wales would be upgraded.

The Guardian reported that the programme would involve installing hundreds of miles of electric cables as well as alterations to tunnels, bridges and stations.

Work would be spread over a decade to minimise disruption to services, it added. FirstGroup (FGP.L) operates the main franchise for trains between London and Cardiff.

Only 40 percent of Britain's rail network is electrified, a figure that lags electrical coverage in many other European countries. (Reporting by Keith Weir; Editing by Elaine Hardcastle)