Electrification of Swansea to London trains 'by 2018'
Passengers will benefit from faster train services between Swansea and London by 2018 when work to electrify the line will be completed.
Network Rail says modernising one of the busiest lines into south Wales will cut 20 minutes off the three-hour journey.
The £350m investment will also see lines from Cardiff to the south Wales valleys electrified.
However those lines may not be ready until about 2020.
The south Wales rail electrification, and the prospect of longer, faster trains, was announced last summer by the Department of Transport.
But Tuesday sees the first official timetable given by Network Rail which will be building the line.
The firm says electric trains powered by overhead cables are cheaper to run than diesels because of lower fuel and maintenance costs.
Much of the work is expected to be carried out at night, with a so-called factory train laying a mile of new track and cables every night once construction is underway.
Mark Langman, Network Rail route managing director for Wales, said: "The plans we have set out will deliver the biggest investment in the Welsh rail network since the Victorian era.
"Much of our infrastructure still dates from that period and this investment will help bring the network into the 21st Century."
Mr Langman said the electrification announcement of the Swansea and Valley lines was made back at the beginning of last summer.
He added: "They're commitments of the government and it's all about now the cost of building it and when we'll be building it."
He said passenger disruption when work starts on the lines would be kept to a minimum.
"It's early days in our planning and we're already working on using new innovations to try and keep disruption to passengers to a minimum," Mr Langman said.
"For instance, with the electrification, we've ordered a new machine that actually that works during the night when the trains aren't running."
He said the machine moved slowly at walking pace along the tracks putting the new equipment in the ground.
Network Rail said passenger numbers in Wales had "grown significantly" over the past 10 years and this was forecast to continue.
Last month, Welsh Secretary David Jones said Wales was benefiting directly or indirectly from almost £2bn modernisation.
"As a regular rail user, I know that the re-signalling of many rail lines across Wales can only be good news for all passengers and I am particularly pleased that the important north-south line links will be some of the first to be upgraded," he said on Tuesday.
"The Newport to Shrewsbury re-signalling also marks the start of the build programme for electrification of the south Wales mainline which we announced last year.
"The confirmation that the mainline route between London and Cardiff will be electrified by 2018, reducing journey times and helping to make Wales a more attractive place to do business, is particularly welcome."
Professor of transport Stuart Cole, from the University of Glamorgan, told BBC Wales the significance of the announcement was the "solid dates" given by Network Rail.
He said: "Network Rail is a cautious organisation. It has given a set of dates on which it has done a lot of work and therefore is convinced that these are set of dates which will work."
He said one of the valley lines will have been electrified by 2017, allowing the train operator with the franchise to begin training drivers on the new trains.
He added: "All that needs to be done on a gradual basis so that by the date that's set out in the strategic plan, say 2020, all that work will have been completed.
"I think we can expect to see the first electric trains in the valley lines around 2017."
The other significant and new elements in the business plan is a timetable of improvements on signalling between Chester and Llandudno for 2015 and also work signalling between Newport and Shrewsbury.
Network Rail said this signalling work could take 15 minutes off journey times between north and south Wales.