Despite the large and small tags, there was actually little difference between the two groups, the most crucial being larger boilers with an increase in tube heating surface.

The class was originally designed by Peter Drummond, whose elder (and better known) brother Dugald had been in the company's locomotive department in earlier years and was at that time Chief Mechanical Engineer of the London and South Western Railway.

In appearance they were fairly typical Drummond family products with the stiffener across the cab roof.

The first 8 locomotives were of the Small Ben type and were built by Dübs and Company of Glasgow in the period 1898 to 1899.

1 was originally named Ben Nevis for its works portrait, an action that attracted the ire of the board as it was in the territory of a competitor (the North British Railway).