The increase in the frame length between the preceding "40 Series" and the "40-2 Series" six-axle locomotives was made to accommodate the new HT-C truck design, in which the traction motors all face the same direction, making the trucks longer. After a series of derailments involving Amtrak SDP40F units that were equipped with "hollow bolster" HT-C trucks, applied only to the SDP40F, Conrail ordered the SD40-2 units and several orders of SD50s with the older Flexicoil trucks, but the HT-C truck was vindicated and it ultimately went under most "40 Series", "50 Series" and "60 Series" six-axle locomotives, and this truck is still found under many remanufactured locomotives.