No. 5972 Olton Hall had its first brush with Hollywood in 2001 when Warner Brothers chose the engine to star in the first Harry Potter film but the locomotive hasn’t always enjoyed such a charmed life. Built in Swindon in 1937, the train was withdrawn from service in 1963 and scrapped. It was rescued from Barry scrapyard in South Wales in 1981 and had a makeover Hollywood would be proud of in 1997.

‘It’s nothing more than a fluke that it wasn’t broken up and turned into razor blades,’ Ian Crowder, GWR’s press officer (and an aspiring train driver) tells me. ‘Olton Hall is a good example of what can be done with a load of scrap iron. Over 200 locomotives ended up in that scrap yard, just engine corpses.’

Many of the components for the train were manufactured from scratch although the wheels, frames and parts of the boiler are likely to be original. It’s the first time no. 5972 will have run on a heritage railway and passengers will enjoy a 20-mile huff and puff through glorious Cotswold countryside.

‘Having this locomotive here is widening the appeal of the railway,' says GWR press officer Ian Crowder. 'It stars in all of the films and while I think people will come along just to see it, hopefully they will enjoy the experience so much they’ll come back.’

So how do you get an A-list steam locomotive to appear on a heritage railway run by volunteers? ‘We simply asked the owner if we could borrow it’ explains Crowder. ‘We pay a ‘steaming’ fee to the owner, which can be anywhere between £200 and £1000 per day.’