Both the bodies and underframes were designed for interchangeability, as had been specified by BR. To achieve this, they were manufactured upon jigs. They had been designed so that the entire body could be replaced during a mid-life refurbishment/reconstruction, and that the replacement body would not be limited to the exact same dimensions either. The underframe area, in addition to its structural role, accommodated all of the propulsion apparatus along with the majority of electrical gear. As a cost-saving measure, the manufacturers were directly to make use of road bus-standard equipment in several areas, including passenger fittings and the general cab layout, along with other areas wherever possible; Unlike the Class 141, which featured automotive-standard wiring for the traction equipment with resulting poor performance, railway-grade wiring for the traction and braking circuits was mandated by BR for both the Class 142 and Class 143 to yield greater reliability.