Every DART in Dublin is being replaced with brand new trains.
Irish Rail is looking for a company to make 600 new electric trains, which will travel all over Ireland.
Dublin will take the first orders, where the current 76 carriage DART fleet is almost 40 years old.
It will take up to ten years to update the country's entire fleet.
Irish Rail has described the investment as the "largest and greenest" ever in Irish transport history.
It also involves the electrification of lines to Maynooth, M3 Parkway, Hazelhatch and Drogheda.
Irish Rail's Jim Meade said the expansion will provide an "even more frequent and environmentally sustainable commuting option".
However, the company has also left the door open to the first of the new trains being hybrid, in case the lines aren't electrified in time.
Anne Graham from the National Transport Authority said purchasing the electric trains "is the first step in the delivery of Dart Expansion".
The expansion aims to make sure there are enough trains to cope with demand from Dublin's growing population.
It will also play a role in Ireland's efforts to greenhouse gas emissions.
Irish Rail expects today's announcement to attract the interest of "virtually every major global train manufacturer".
It says the project will ensure customers benefit from up-to-date facilities and technology.
