It was the first of a series of broadly similar classes used principally for suburban passenger services.

The development is somewhat convoluted to follow, because of various renumberings and gaps in the number series as listed below.

Fitted with the standard number 2 boiler running at 200 psi (1.38 MPa), flat topped tanks and driving wheels of 5 feet 8 inches (1.727 m) in diameter, it was the forerunner of 289 similar locomotives that were to follow.

Source:[1] The 3150 Class was a 1906 version of the 3100, fitted with the larger and heavier Standard 4 boiler.

The 1931 6100 Class were more powerful versions of the 5101, fitted with a version of the Standard 2 boiler with higher working pressure, giving the locomotives greater tractive effort.

[1] Whatever the practical advantage in performance, the class became widely dispersed and locos were used alongside their predecessors indiscriminately.