The Es class were the result of superheating the standard E class which was built for express services. The superheated boilers were built by Midland Railway Workshops who fitted them to the E class when they went in for repairs. In their superheated form, the engines consumed less coal and were more cost-efficient on heavier trains. Some were fitted with larger cylinders and classed Esl, those who retained original cylinders became Eso - however, these designations were later dropped, becoming simply the Es class. No. 308 has been preserved and is on display at Bassendean Railway Museum.
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