Characteristic features were belpaire fireboxes, snowplough like cowcatchers, smoke deflectors and six wheel tenders.
The first in the series of standard engines were the mikados.
Their design was chosen when the people in charge of the project leased several kinds of mikados. Testing them all to see which was the best performer.
In the end, it was decided to use the Chicago, Burlington, and Quincy O class mikados as a model for the new engines.
The first standard mikado was built in 1961, and tested successfully on the Norfolk and Western over its mainline between Roanoke and Bristol.