Crossrail is paying 140 drivers up to £62,000-a-year to operate trains despite the heavily-delayed line not opening to passengers for another 18 months.
The employees, who are working four days a week, are running tests on the tunnels through central London and outside the capital.
It comes despite 'intensive trial running' - which will be like the line when it is fully open - not starting until next year.
The news will stoke anger after it was revealed this week Crossrail was given an extra £825million to prevent the delayed project from being mothballed.
Transport Secretary Grant Shapps said the loan would help 'get the project up and running' following years of delays and spiralling costs.
The railway - which will run as the 'Elizabeth Line' from Berkshire to Essex via central London - was originally due to open in December 2018.
The project's budget was set at £15.9billion in 2007 but has been hit by spiralling overspending since then and is now predicted to cost £18billion.
The employees, who are working four days a week, are running tests on the tunnels through central London
The news will stoke anger after it was revealed this week Crossrail was given an extra £825million to prevent the delayed project from being mothballed
TfL said the train drivers are important for the tests and they were being ramped up as part of safety preparations.
The local government body said training the workers will take two years because it is being done from scratch.
As many as 435 drivers - excluding trainees, supervisors or people shielding from Covid-19 - can be used by Crossrail, new figures show.
But 295 are working on parts of the line currently running between Essex and Berkshire.
It leaves 140 that 'are available for supporting dynamic testing of the new central operating section', according to the Times.
Transport Secretary Grant Shapps confirmed a deal with Transport for London (TfL) that would see ministers stump up a £825million loan in order to complete the crucial middle section of the east-west route under the capital
This had begun as two trains working simultaneously, but it is understood to have been increased to eight before being further ramped up before 'trial running' next year.
Crossrail drivers pay is fixed at £62,533 a year, union Aslef said, starting at £46,900 for the first year and £31,124 for trainees. A two per cent pay rise is set for April.
A TfL spokesman said: 'All available drivers are driving TfL Rail trains on routes out of Paddington and Liverpool Street that will become part of the Elizabeth line, undertaking vital testing of the central section or training on other sections of the railway.'
An update from Crossrail Ltd at the end of August said the opening of the railway was now not expected until the first half of 2022.
Project leaders recently raised fears the entire scheme could be mothballed following a sharp drop in revenue for Transport for London (TfL) during the pandemic.
The railway - which will run as the Elizabeth Line between Berkshire and Essex via central London - was due to open in December 2018
Andy Byford, the London transport commissioner, warned of the 'Doomsday scenario' in a letter to the Department for Transport last month.
Mr Shapps said the new loan shows ministers are 'committed to getting Crossrail delivered' and described it as a 'far deal for taxpayers across the UK'.
Crossrail chief executive Mark Wild said: 'Delivery of the Elizabeth line is now in its complex final stages.
'Good progress continues to be made with completing the remaining infrastructure works so that we begin intensive operational testing, known as trial running, at the earliest opportunity in 2021.
'Many of the stations are now nearing completion and we will shortly commence an enabling phase for trial running which allows testing in the tunnels to be undertaken with an increased number of trains, further helping to build operational reliability.
'We are doing everything possible to deliver the Elizabeth line as safely and quickly as we can.'