The use of the HT - C truck on most SD - 40 - 2's made a HUGE DIFFERENCE in the performance of the unit. It has been opined by several authors, including Joe Strapac, that a Forty - 2 with 3000 H.P. would out pull a non Dash - 2 SD - 45 with 600 more H.P. !!! Fortunately for the nation's railroads, this improvement in performance AND fuel economy, from the 16 cylinder engine, occured just as fuel prices spiked. This is why, after seemingly every railroad bought SD - 45's, comparatively few SD - 45 - 2's were built. Unfortunately for the railroads, retrofitting an HT - C truck under a non - Dash 2 unit would be quite expensive, as the center plates are of different diameters, and the traction motor blower ducts are arranged differently.
As Conrail ordered their SD - 40 - 2's from the factory with the standard Flexicoil truck, EMD was able to substitute the different center plate on the assembly line with minimal cost.
The use of a radiator sight glass in a long hood door actually pre - dated the Dash 2 series, as Espee ordered the feature on many of its' SD - 45's, as well as its' SD - 39's.