The original aqueduct pictured was built by John Rennie between 1797 and 1801. The stone used came from a nearby quarry but was of very poor quality and the structure soon began to deteriorate, a noticeable 'sag' appearing in the middle arch. Repairs were made using stone from a different quarry and lasted until around 20 years ago when a major programme of restoration work was carried out by the Kennet and Avon Canal Trust. Prior to that work the aqueduct was relined with a long concrete 'cradle' to put an end to leaks from the canal bed when this section of the K&A was restored to use in the early 1980s. Avoncliff aqueduct is a magnificent structure, often overlooked in favour of its near neighbour Dundas Aqueduct just two and a half miles futher on towards Bath.
The railway passes through the canal embankment at the north end of Avoncliff aqueduct by way of a separate brick-built twin arch aqueduct. This replaced an earlier wooden underbridge in 1885 when the line was doubled between Bradford on Avon and Bathampton Junction (see photos already on the site in the Bristol to Bradford on Avon section).
Kind regards, Paul.
P.S. I am advised that Cornish for passengers is 'trethyas'.