From 1927, the then chief engineer of the GWR, Charles Collett, procured 100 pieces of an improved version of the Class 4500 developed by Churchward. The most important difference concerned larger water tanks, which could be visually recognized by the sloping top at the front. Since the company numbers of the first series followed the 75 units of class 4500 without gaps, they became class 4575. The total of 100 units were initially assigned numbers up to 4599 and finally from 5500 to 5574.

Although these locomotives were built later than their predecessors, they remained in service little longer. Their retirement began in April 1956 and was completed in December 1964. A total of eleven pieces still exist today. The number 5521 even came to Poland from 2007 to take part in the annual parade of the museum in Wolsztyn. After that it spent some time in Hungary to pull the Orient Express and after another stay in Poland to come back to England.