Why were direct-current series motors used in subway trains and fork-lift trucks?

Direct-current (DC) series motors were historically favored for applications like subway trains and fork-lift trucks because of their specific torque-speed characteristics. In a series motor, the field coils carry the same current as the armature. This configuration results in high magnetic flux when the current is high, which generates the high torque necessary for starting heavy loads, such as a train or a loaded fork-lift.1

As the motor gains speed and the current draw decreases, the magnetic flux also drops, which allows the motor to increase its speed. This inherent ability to provide high torque at low speeds and higher speeds under lighter loads made them ideal for the variable demands of electric transportation and industrial lifting equipment.1