The Ec 3/3 was ordered when the shunting duties in large stations and yards grew. Numbers 301 to 306 were delivered by SLM in 1897, followed by 307 to 312 in 1901. With a service weight of 42.7 tonnes, they were the heaviest shunting locomotives in Switzerland at that time. They had drivers of 1,230 mm and cylinders which measured 380 mm in diameter and 600 mm in stroke. Their water tanks had a capacity of 4.5 tonnes and were sloped at the front to increase visibility. Although not designed for line service, they had an air brake and were equipped for steam heating.