From Sunday, the line from Treherbert to Pontypridd in Rhondda Cynon Taf will close and replacement buses introduced.

The work is part of the shift to the South Wales Metro, which is now £260m over budget at £1bn.

Transport Minister Lee Waters said the disruption would be "worth it", but one NHS worker said replacement buses set off too late for his job.

Signalling technology dating back to the 1930s is among the old equipment being replaced and people living in Rhondda Cynon Taf will get half-price tickets while the work is happening.

However, this discount only applies for trips only taken on this line, down to Cardiff Central.

New equipment, masts and wires are being fitted to modern electric trains can use the tracks, which also means a number of gas and water mains will need to be diverted.

Some station platforms will be replaced and new footbridges will also be installed.

Leanne Culverhouse says the "uncertainty" around replacement buses makes her worry

About 12 miles (19km) of track will close, but Transport for Wales (TfW) has not committed to an exact date when it will reopen.

Passengers and businesses alike have said they will be badly affected.

NHS worker Iestyn Tenko travels from Caerphilly to Treorchy each day and is worried the replacement bus will not get him to work on time.

"I'll be arriving 10 to 15 minutes late each day because the rail replacement bus hasn't provided an early enough service.

"I'll have to speak to my manager to see if I can start later, otherwise I'll have to find a new job local to home."

Tetiana Mokhova is considering buying a car as she does not have faith in the buses to get her to work

Leanne Culverhouse relies on the train from Ton Pentre to Pontypridd to get to work as a doctor's receptionist.

She said it would be "a long 10 months" and was worried the bus would be late "or not turn up".

When the closure was first announced in March, TfW expected it to last eight months. It is now predicted to last 10.

TfW's Lowri Joyce said the work was "a significant part of our plans for the South Wales Metro so over the next 10 months, that's where our focus will be. It's a complex piece of work".

Ms Joyce said the end result would be "newer trains, faster trains, more frequent services - it's an upgrade that's been needed for a long time".

Sonal Patel fears for her business, which relies heavily on train passengers

Sonal Patel, who owns a convenience store near Porth station fears her business will be devastated as about 80% of her customers use the trains.

"If it is closed it's going to affect my business totally," she said.

Beautician Tetiana Mokhova commutes to Treforest by train but is considering buying a car.

"It will not be easy but I have to because it will be easier. The bus is not so reliable. They can stop running without reason," she said.

Factory worker John Jenkins travels every day from Treorchy to Mountain Ash.

He accepts there will be disruption, but is pleased that improvements are being carried out.

"It might take a bit longer, but I just go with the flow I'll leave earlier if I have to. I think it will improve it."

Welsh Conservative transport spokeswoman Natasha Asghar described said the 10-month wait was "unacceptable", with alternatives being "a crumbling road network and an under-funded bus network".

Plaid Cymru South Wales Central Member of the Senedd Heledd Fychan said: "It's crucial that the rail replacement scheme is a success, with lessons learnt from previous work."