TOMORROW night, I'm giving a new talk at the Friends of Darlington Railway Museum agm. They are lucky, lucky people whose insomnia should be cured.

It is called "Rockliffe and the Railways" and comes out of the research I've been doing on the Backhouse mansion in Hurworth which is now Rockliffe Hall.

My contention is that one of the reasons Alfred Backhouse bought that estate in 1860 was because its western edge was framed by one of the world's first stretches of mainline which, of course, his family had financed. It is almost as if Alfred, and his renowned archiect Alfred Waterhouse, designed the mansion so that it had views of the Tees Viaduct - even today, the viaduct looks breathtaking as a backdrop, but it was far enough from the house for the steam and stench not to take the breath away.

Anyhow, finishing touches for the talk have led me down the path of looking into the first engine to traverse the viaduct on the day that it opened: January 4, 1841. Two locos, No 26 Pilot and No 27 Witton Castle, pulled the first train from Darlington to York over the viaduct. Two more locos, No 24 Magnet and No 15 Tory, pulled it back again.

Magnet was the oldest of these engines, built by Timothy Hackworth at the Soho Works in Shildon in July 1835 for £1,050 (£100,000 in today's money, according to the BoE inflation calculator). Magnet was one of a kind, the most powerful engine of its day, but it had disappeared from service by 1846.

The three other engines were slightly later and from the "Tory class". The class was named after the first engine of the kind to be built, in this case No 15 Tory which was again built by Hackworth at Soho and delivered in November 1838. Six months later, its boiler blew up at Preston, Stockton, blasting the boiler end 50 yards, and so the Tory that trundled across the Tees Viaduct was a rebuilt engine.

The engine name does not indicate a political bias. The next engine was No 17 and it was called Whig, to keep the balance right.

The other two Tory class engines on opening day were the newest in the S&DR's fleet. No 27 Witton Castle was built by Neasham & Welsh in East Portrack Lane, Stockton and started work in May 1840. No 26 Pilot was delivered from William Kitching's foundry at Hopetown, Darlington, in June 1840.

So now I know about the first engines across the viaduct. But what did they look like? Only a picture of Magnet survives from a timetable, but in the Head of Steam there is an actual survivor if the Tory class: No 25 Derwent, delivered in November 1845 by Kitching's.