It is one of the world’s most iconic railway journeys, offering passengers a trip over the curved Glenfinnan viaduct made famous by the Harry Potter films.

But the service known as the Hogwarts Express has been hit by delays as its operator awaits the green light to resume its services.

West Coast Railways, which runs the service that is officially known as The Jacobite and covers 41 miles between Fort William and Mallaig, had been ordered by the Office of Rail and Road (ORR) to replace its 70-year-old carriages with more modern stock with central door locking.

However, the rail firm - said to be the biggest operator of steam trains on the national network - had flagged the newer carriages as unpopular with passengers, and also required adding a diesel carriage to fuel an energy-hungry air conditioning system.

West Coast said such an addition was expensive and also reduced the journey’s number of passengers due to a limit on train length on the West Highland Line. The older trains also have windows that open, unlike the newer trains, which depend on the air conditioning system.

James Shuttleworth, commercial manager at West Coast Rail, told the Scottish Daily Mail that the firm has submitted the paperwork to the ORR to be able to resume using the 1950s carriages if it implements central door locking. This process, which will introduce a centrally controlled automatic lock controlled by the guard, could cost more than £5million to cover the 120 carriages.

The firm is awaiting an exemption to run the old carriages as it implements the new system, and hopes to receive this nod within weeks.

 However, the service that began operation in 1984 was meant to resume this year on April 1, and would normally be carrying 350 passengers on its one daily journey in April, doubling this in May. 

Tickets for adults cost from £69 in standard class, £105 in first class, and up to £425 for a compartment, while the train’s crew have been redeployed elsewhere in the business until the service resumes.

‘The demand is there - I’m hopeful that we will get a positive response in the not-too-distant future from the ORR,’

Mr Shuttleworth said, adding that the service can resume immediately if so. He also expects to see renewed demand for the service when the new Harry Potter TV series debuts later this year. The trailer was unveiled by TV network HBO last month.

The rail journey also brings much business to many firms en route. John Barnes, manager of the Glenfinnan Station Museum, said the delay to the train service resuming had reduced visitor numbers at the attraction, which ‘seeks to tell the story of the West Highland Railway, its construction and development’.

Mr Barnes added that the lack of certainty over when the train will return makes it difficult for the museum - which he believes JK Rowling herself visited some time ago - to plan staffing levels. It gets an ‘enormous amount’ of footfall from the train, ‘but we’re only a small museum, so we don’t notice [the drop] as much as larger attractions’. The museum opened in 1991 and also has a restaurant and dining car.

The ORR was quoted saying proposals for the train remain under assessment. A spokesman added: ‘We note that West Coast Railways recognises the need for central door locking on the [older] Mk1 carriages it operates. However, work remains to establish the feasibility, detail and delivery timeline of the proposed approach.’