Rigid-framed electric locomotives were some of the first generations of electric locomotive design. When these began the traction motors of these early locomotives, particularly with AC motors, were too large and heavy to be mounted directly to the axles and so were carried on the frame. One of the initial simplest wheel arrangements for a mainline electric locomotive, from around 1900, was the , in UIC classification. By the middle of the century, the bogie arrangement for locomotives became more popular and rigid-framed locomotives are now rare, except for small shunters.