Thomas Wheatley became locomotive superintendent of the North British Railway (NBR) at the start of February 1867.

[7] During his tenure of seven years, he provided the NBR with 185 new locomotives;[7] but only eight of these were suitable for hauling express passenger trains, the first two of which were 2-4-0s built in 1869 (the 141 Class),[8] which were considered to be very good engines.

224 was the first inside-frame inside-cylinder 4-4-0 to run in Great Britain, and predated the G&SWR 6 Class[4] by some two years, the latter being introduced in July 1873.

224 was used in Fife, which in the days before the construction of the Forth Bridge, was an isolated part of the NBR system.

[20] They were intended for use on the Waverley route, over which an increase of traffic was anticipated: the Midland Railway (MR) were at the time building their Settle–Carlisle line.

This route not yet being open, and the English traffic being entirely in the hands of the London and North Western Railway which worked closely in tandem with the Caledonian Railway, the NBR's main rival, the trains over the Waverley route were comparatively light and well within the capabilities of the 420 Class.

[21] The MR opened the Settle and Carlisle line on 1 May 1876, and a through service using that route between London St Pancras and Edinburgh Waverley was introduced at the same time.

But these locomotives proved insufficiently powerful, a second engine often being needed to assist in climbing the gradients,[25] particularly those at Falahill (between Tynehead and Heriot) and at Whitrope (between Shankend and Riccarton Junction).

[24][26] Wheatley's successor, Dugald Drummond, offered the opinion that NBR express locomotives of the period were "like skinny chickens, all legs and wings".

[29] Until the opening of the Forth Bridge in 1890, passengers from Edinburgh to Dundee would cross the Firth of Forth by a ferry from Granton which connected with trains at Burntisland.

[30][5] Further north, the Firth of Tay had been bridged in 1878, and trains could travel through from Burntisland to Dundee and onward to Aberdeen.

[5] It did so without incident on the southbound run, but when working the 5.20 p.m. northbound service later in the day, due to arrive at Dundee a little before 7.30, it was on the Tay Bridge when shortly after 7.13 p.m. the latter collapsed.

224 had no warning of the impending disaster, and neither closed the regulator nor applied the brakes;[34] they were among the 75 people killed.

[35] Despite the fall, the locomotive was relatively undamaged, being protected by the bridge girders which formed a cage around the train as they fell together.

Two days later, a second attempt also failed because the salvage equipment broke after the locomotive had been brought to the surface.

[37] One week later, it was recovered, and stood on the bank of the Tay until it was sent to Cowlairs on its own wheels for repairs, after which it was returned to traffic.

[44] The boiler pressure was 140 pounds per square inch (970 kPa), and the grate area 16.6 sq ft (1.54 m2).

[10][42] Although not entirely successful,[19] it did somewhat better than the only two other British tandem compounds – Great Western Railway nos.

[19] Every six months, the NBR renumbered some of its older locomotives into a "duplicate list", in order to vacate numbers for new construction.