* Plan includes electrification of some lines
* Investment adds 4.2 bln pounds of new projects
* Building work to start by 2014 at the earliest
LONDON, July 16 (Reuters) - Britain is to invest 9.4 billion pounds ($14.6 billion) upgrading the country's rail network, including 4.2 billion on new schemes such as reopening a line that will provide better access between northern Britain and ports on the south coast.
Other new projects include electrification of lines in the Welsh valleys and the London-Sheffield service, and a 500 million pound link that will better join London's Heathrow airport to the national rail network.
Building work on the projects, which the government predicts will deliver capacity for an extra 140,000 daily commutes, will not start before 2014 -- the announcement covers the period 2014-19.
"This investment will mean faster journeys, more seats, better access to stations, greater freight links and a truly world-class rail network," Prime Minister David Cameron said at a news conference in Birmingham.
The plan includes 5.2 billion pounds for the completion of current schemes, such as the all-new Crossrail that will cross London east-west, and an upgrade of Thameslink, which runs north-south through the capital.
The government said the plans would be funded, in part, from previously announced fare rises and from efficiency savings electrification will have on operating costs.
The Federation of Small Businesses said the investment was long overdue and would help small businesses better connect with customers and suppliers.
"We have long been calling for better road and rail links to be put in place so this will be welcomed by the business community," FSB chairman for infrastructure Neil Hoose.