After introducing two unsuccessful designs of
4-4-2 tank locomotive with the
I1 and
I2 classes,
Douglas Earle Marsh learned a lesson and provided a new design with a far larger firebox. The new design was a tank version of
Robert Billinton's successful
B4 class tender locomotives. At the time of its introduction locomotive engineers were beginning to take an interest in
superheating and Marsh therefore ordered two locomotives from
Brighton railway works for comparative purposes, one with a traditional saturated boiler and one incorporating the
Schmidt superheater. These were built in October 1907 and March 1908 respectively. Several months of trials proved that both locomotives were performing well and reliably, but the LB&SCR Board of Directors were still not convinced that the extra building costs associated with superheating was matched by lower running costs. Marsh therefore built four more superheated locomotives and six without superheating, between February 1909 and March 1910. By mid 1910, there was sufficient operating data to convince the Directors and all after this date were superheated. Five more locomotives appeared in 1910. Both varieties of the I3 class proved themselves to be excellent locomotives but the superheated versions were significantly cheaper to run on express trains, especially when compared with the
B4 and
H1 classes. The class were also used experimentally on through express trains with the
London and North Western Railway between
Brighton and
Rugby and convinced the LNWR directors of the value of superheating. Ten more superheated I3s were constructed in 1912 under
Lawson Billinton, with minor detail differences. After World War I, Billinton wished to convert the remaining members of the class to superheating as their boilers came due for renewal, but only the prototype No. 21 was converted before the LB&SCR merged with other railways to form
Southern Railway in January 1923. The remainder were converted by
Richard Maunsell between 1925 and 1927 using his own design of superheater. During 1925 and 1926, the I3 class were gradually replaced on the London-Brighton express trains by the
"King Arthur" and
"River" classes, and they were transferred on to semi-fast and other secondary services. However, the electrification of the London to
Brighton and London to
Portsmouth lines during the 1930s meant that the class were transferred further afield on the
Southern Railway including
Salisbury and
Dover. Between the Autumn of 1941 and the summer of 1943 two I3 locomotives were loaned to the
Great Western Railway and performed well on services from
Gloucester, and
Worcester. ==Accidents and incidents==