Dugald Drummond's first design for the LSWR was the class M7 0-4-4T tank locomotive, which was the larger successor to Adams' T1. While the driver diameter remained the same, over all the grate was enlarged, resulting in the heaviest British machine with this wheel arrangement. It was intended not only for suburban lines in London, but also for semi-fast trains. After a derailment at higher speeds, they were only used on suburban lines.

Between 1897 and 1911, 105 locomotives were built, differing in several details. These included different frame lengths, different arrangements of individual components such as the sandboxes and the reverser, which was initially a lever and later steam-operated. To reduce wear and tear, boiler pressure was reduced from 175 to 150 psi after high speed running on main lines was no longer necessary.

With the introduction of new locomotives on suburban lines, the M7 was increasingly used on branch lines. For this purpose, a larger number were equipped with push-pull controls, which initially worked using cables and pulleys. Later, a more reliable system that worked with air pressure was used. The retirements took place in greater numbers from 1958 onwards and were completed in 1964 after the M7 was replaced by modern standard steam locomotives, diesel locomotives and diesel multiple units. Two still exist today, of which 30053 is operational. It was based in Steamtown in the USA between 1967 and 1987 and is now stationed on the Swanage Railway.