The last of the initial A1 order, Engine Order No. 382, 60123 was the tenth class member built at Doncaster. Its works number was 2040. With Darlington-built 60142 also entering service on February 10th 1949, they were the 22nd and 23rd A1s to come into traffic. Of note is that 60123 was the first A1 to be shedded at Doncaster. Livery of this plain-chimneyed Pacific was the customary apple green with black and white lining and the nationalised railway's name carried on the tender in white block capitals. The first working recorded was a down passenger train into Newcastle then a visit to Gateshead shed, both on June 16th. After only ten months in its original livery, a repaint into BR express blue was made in December. This was the fifth of the 39 A1s delivered new in green to be repainted. A transfer to Grantham shed on March 19th 1950 kept its area of operations on the English section of the East Coast Main Line; 60123 was seen at Newcastle on March 25th and at Darlington on April 23rd.

A transfer to Copley Hill shed on June 4th shifted 60123's operations to Yorkshire and the lower section of the main line. In fact it was to remain a Yorkshire-based engine for the rest of its life. Naming in July as H. A. Ivatt commemorated an eminent Great Northern Railway CME, one of three from that railway carried by A1s and one of half a dozen class members named after East Coast engineers. This locomotive was among the earlier A1s to be named with another six so treated that month while another nine had been named earlier. A special train was the return Ivatt Atlantic special to Kings Cross which on its outward trip was hauled by the last Ivatt 4-4-2 on its final run, 62822, on November 26th. Unfortunately, an extra fast schedule was spoiled by an over-heated axlebox on a restaurant kitchen car, causing a considerably delayed arrival with 60123 at the terminus. Other changes were the fitting of a lipped chimney and the removal of the Flaman speed recorder.

Yet another reallocation came on September 9th 1951 with a move to Ardesley shed. H. A. Ivatt's first named trains were the down "Queen of Scots" on May 25th 1952 from Leeds - Kings Cross and the up train as far as Leeds on July 26th. Between October and December, 60123 along with 60122 and 60135 was experimentally fitted with an inside connecting rod with a Swindon-style forked big end with a clip and cotter arrangement to secure the brasses. At the conclusion of this trial in 1956 it was deemed to have been a success. H. A. Ivatt was one of the last A1s to be repainted into BR green; it was one of three done in December 1952 after 43 had already been dealt with. The start of the next year found 60123 as one of ten class members hurriedly fitted with the Hudd system of ATC. Of note are two special; a Rugby special into Kings Cross on April 25th and a Cup Final special to the same destination on May 2nd. A less common working was the 6.05 a.m. Kings Cross - Cambridge on July 15th 1954. In the mid 1950s the smokebox numberplate and handrail were transposed. Detailed logs at Kings Cross between September 1956 and January 1957 show 60123 primarily on trains to Leeds with the 9.18 a.m., 1.18 p.m. and 3.40 p.m. departures noted but by far the most common turn was the 6.15 p.m. Several noon departures with the "Queen of Scots" were noted plus the 7.50 a.m. Kings Cross - Newcastle / Bradford and the 8.20 p.m. to Edinburgh. On November 17th 1956 H. A. Ivatt was seen at Bradford shed under repair. Other named trains hauled included the down "West Riding" on June 6th and 22nd 1953 and the down "Aberdonian" from the capital on September 26th 1956. Observation on June 1st 1957 of a Kings Cross - Leeds train hauled via Lincoln was presumably due to diversions.

60123 H.A.Ivatt on a down express north of Potters Bar on 21st May 1955 - Robin Gibson

A transfer back to Copley Hill was made on September 15th 1957. The following January the later BR crest was applied to the tender. Trains to and from Yorkshire included the 8.21 a.m. up "West Riding" on June 27th 1959 returning with the 4.28 p.m. to Leeds; the 11.15 a.m. up "Yorkshire Pullman" on August 8th; then on June 24th and July 1st 1961 the 8.21 a.m. up "West Riding" returning on the 4.20 p.m. down "Tees-Tyne Pullman". A hockey special was hauled from Leeds to the ‘Cross on March 10th 1962. Around this time a Smith-Stone speedometer was fitted to one of the rear driving wheels.

Transfer back to Ardesley on April 1st 1962 presaged the last stage of 60123's life. With six allocations it had been moved around more than most A1s. Passenger trains continued much as before, examples being the down "Tees-Tyne Pullman" from the capital on August 8th and the 3.55 p.m. to Leeds on the 18th. The first recorded non-passenger working of 60123 also proved to be its last. On September 7th it suffered severe damage when it derailed at Offord when pulling the 8.20 p.m. Kings Cross Goods - Leeds. It entered Doncaster Works on the 24th but was withdrawn on October 1st - the first a1 to be condemned. With a service life of just 13 years 7 months, 60123 was one of the shortest-lived A1s. In its time it had carried seven boilers. Scrapping was at Doncaster Works; the last reported sighting of 60123 was in the Works yard on October 14th.