The Class 125 was built in 1958 at Derby Works as high capacity suburban railcars specifically for the Lea Valley Lines near Stratford.

They were fitted with powerful Rolls-Royce engines, allowing them to have comparable performance to contemporary EMUs.They bore a close resemblance to the similar class 116, which were also built by British Rail at Derby.

The trains were then transferred onto the East Coast Main Line commuter services out of King's Cross, where they worked for the rest of their operational lifetimes.

The whiskers were replaced during the early 1960s by split yellow warning panels, one either side of the central character train describer which remained green.

During the mid 1960s rail blue appeared, and white cab roofs were gradually dispensed with and buffer beams became black.