The DD35 Demonstrator pack, which requires the Union Pacific DD35 Add-On to use, enables you to drive the Gp35 and DD35 as they appeared in 1963 when being demonstrated on various railroads but only Union Pacific and Southern Pacific took on the units.
The DD35 was a cabless booster (B unit) locomotive that consisted of two GP35 prime movers and electrical systems, mounted on a common frame and covered with a single cowling, riding on a pair of 4-axle Flexicoil trucks. As such, each prime mover powered a single set of four axles instead of two sets of two axles, as in the GP35.