Commuters will now be able to travel from Reading to Paddington in 58 minutes, after a limited section of Crossrail opened yesterday.
The new line, under the banner of TfL Rail, marks the largest opening of any part of the Crossrail project, which was supposed to open in December 2018.
It will be renamed the Elizabeth line once Crossrail is fully opened.
The TfL Rail commute from Reading to Paddington will also include stops at Twyford, Maidenhead, Taplow, Burnham, Slough, Langley, Iver, West Drayton, Hayes & Harlington, Southall, West Ealing, Ealing Broadway and Acton.
Tfl Rail operations director Howard Smith said: “This is an important step towards opening the Elizabeth line.
“Customers will notice continual improvements along their route including a new Sunday service at Iver and Taplow and improved accessibility at several stations with lifts being installed to provide step-free access from street to platform.”
The project, with an original budget of £15bn, is expected to go over budget by up to £2bn.
Read more: Crossrail delays may cost London £2bn
Transport for London (TfL ) predicts the central part of the line should open by 2021.
When fully operational it will link Reading and Heathrow to Shenfield and Abbey Wood.