Video report from Caerphilly where the new Class 231 trains were officially launched
How do you build a new railway while continuing to operate the existing one?
Passengers living on the routes of the core valley lines have become used to regular closures as track and electrification work has been carried out.
But the recent announcement that the Treherbert line will close for the rest of the 2023 goes much further.
Passengers living in the Rhondda will be entitled to a 50% discount on the replacement bus services that will operate.
Mr Price told ITV News that the Treherbert Line is the "one that has the most difficult set of conditions of all the lines we have worked on, and it also has some of the oldest infrastructure of anything that we've worked on."
He added: "We've cabling that has to be patched up every night, we've got services in the way - gas and water - we've got steep sided valleys.
"In essence, it is sadly not possible to transform that line without closing it whilst we work.
"The plan is not for it to be anything like a year. The team have done an awful lot of due diligence, they understand what they're doing, and everyone will be fully incentivised to get that line open.
"The really important thing is once that line is open, all the work is then done, and we can start to run the new rolling stock on it very quickly - in fact quicker than if we had to find a way to do it with a series of night closures."
With the target dates for completion of the project slipping to the 2024/25 period, Transport for Wales stresses the progress that has already been made.
The first of its brand new Class 231 FLIRT trains are already in use on the Rhymney line, promising better capacity and greater comfort.
Its depot and control centre at Taffs Well is under construction, and has taken delivery of the first tram-trains that will run on the Aberdare, Merthyr and Treherbert lines to Cardiff.
Inflation and delays to the scheme have caused the cost to rise to around £1bn.
The Welsh Government insists it is committed to the flagship project.
So, will the sharp increase in cost force a scaling back of promised plans?
"The cost of concrete has gone up, the cost of steel has gone up," the Deputy Minister for Climate Change, Lee Waters MS said.
"There's 1000 people working on this project.
"We've had delays because of Covid means extra wage bills for those people.
"It has consequences, and it may have further consequences. And we're going to have to look at the way we stage the rollout against the original plan.
"But we we are rolling out the Metro, and we are rolling out new trains right across Wales.
"We know the consequences of the disruption, and we know the prize for getting it right - getting more people on the railways and improving people's lives."
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