Though the original rebuilds of the Class As had reused the existing small (4 ft 3 in diameter) boilers with 19.5 in diameter cylinders (Class C, the smaller boilers could not raise adequate steam, so from 1906 the next 62 rebuilds (63 according to the LNWR Society) were rebuilt with a larger 5 ft 2 in diameter Experiment-type boilers, retaining the cylinders.

To these compound rebuilds was added the prototype eight-coupled goods engine No.

Previously it had been similar to the Class C with a smaller boiler; though was not classified as such since the letter classification was introduced in 1911.

All passed into LMS ownership in 1923, and the LMS assigned them the numbers 9002-64, sequentially in order of rebuild date.

The LMS rebuilt all of the Ds with the addition of superheating, taking them into Class G1, nicknamed "Super Ds" between 1925 and 1934.