In 1912,
Nigel Gresley replaced Ivatt as chief mechanical engineer for the GNR. That same year, he introduced a new 4 ft 8 in boiler for the D2s and the
GNR J5 0-6-0s. Although the firebox was shorter, the overall area of 105 square feet was two square feet more than the D2s. The first member to be rebuilt was No. 1359 in November 1912, six months after J5 No. 1163. The rebuild also involved shortening the chimney and raising the boiler. By 1923, six as-built D2s (now classified as D4s) remained. The last member to be rebuilt was No. 4358 in June 1928. In 1923, No. 1305N was rebuilt with slightly larger tubes and a larger firebox. In this form, it became part of the LNER D2 Class, which had seventy other previous locomotives built between 1898 and 1909. In 1926, No. 4320 was also rebuilt, bringing a class total of seventy-two D2s. The D3s meanwhile had forty-nine members left in service. In the 1930s, the Stainmore Route in County Durham, which ran from Darlington to Penrith, was in need of more locomotives to replace the
LNER D23 Class 4-4-0s. Initially
LNER J21 Class engines had been tried, but these tended to have motion failures, particularly after descending the line's banks due to a habit drivers had when it came to driving the class in these areas. It was decided to test D3s on the line to see if they would perform better. To do so, they were fitted with larger cabs and side windows for better crew protection. However, the class proved to be unsuited for the Stainmore line. In 1944, No. 4075 was fitted with a
Thompson cab and, after World War II, was painted in LNER apple green, the only member of her class to carry this livery. ==Withdrawals==