Opened in 1812, and was the first railway in Scotland to obtain an authorising Act of Parliament; it would soon also become the first railway in Scotland to use a steam locomotive; the first to carry passengers; and the River Irvine bridge, Laigh Milton Viaduct, is the earliest railway viaduct in Scotland (see separate entry) and first on a public railway.
The line was engineered as a plateway at a time when other Scottish lines were using the edge rail. The choice of the older technology may be due to the employment of William Jessop as engineer. He had built the Surrey Iron Railway, also a plateway. The wrought iron rails were L-shaped, and the upstand guided the wagons; the wagon wheels did not have flanges, which enabled them to be easily moved around ter-minal areas where there was hard standing, and to and from locations further from the railway. The plates were three feet long, with a four-inch width and a three-inch upstand (920 mm long by 100 mm by 76 mm). Highet says, "These plates or rails were joined with a square joint and were nailed to the foundation stones through small square holes formed half-way in each end of the rail." Clearly the plates were carried on sleeper blocks; the "nails" refers to pins fastened into timber plugs in holes drilled in the stone blocks. The gauge of the line (i.e. the dimension over the upstands) was 4 ft (1,219 mm). The route was nine miles six furlongs (15.7 km) in length.

Passenger traffic had not been contemplated when the line was conceived, and as a toll railway, it was not for the Railway Company to introduce such trains. However William Wright of Kilmarnock operated an irregular passenger service. He paid the Railway Company a toll based on tonnage of passengers; this practice continued up to 1839.
In 1817 the Duke of Portland acquired a locomotive for the K&TR named The Duke, which was the first use of steam locomotive power in Scotland. Its use was however discontinued in view of the frequent breakages of the cast-iron rails, by which the working of the line was interrupted, and accordingly horses were again employed as before.
At Kilmarnock, the terminal fronted St Marnock Street, now Portland Road, west of Dundonald Road junc-tion and immediately west of the Holy Trinity Episcopal Church. There are plaques on what is now the Procurator Fiscal's Office detailing the original site (see gallery). In 1846, when more modern railways had developed through-out the West of Scotland, the line was converted from a plateway to a railway and realigned in places, including Laigh Milton where the original viaduct was replaced. The line became part of the Glasgow & South Western Railway system running from the present station.
Most of the present Kilmarnock to Troon railway line is on the K&TR alignment. The original viaduct fell into decay until restored in 1995. At Troon, the alignment can be inferred from satellite images, but little tangible remains, other than street and building names (Duke's Road, Portland House, Titchfield Cottage).

Description from Pioneering Scottish Railway - with thanks

The Red Wheel Plaque was unveiled on 19th November 2021 by Willie Coffey MSP accompanied by NTT Vice President John Yellowlees and Deputy Charman Jerry Swift.

Platform photo: Rosser1954, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Railway Viaduct at Kilmarnock Station:
cc-by-sa/2.0 - © Gordon Dowie - geograph.org.uk/p/1437165