They are generally used in yard duty where the switcher has enough horsepower, but not enough tractive force to push long strings of cars up a hump. However, a road slug is used in transfer service where the top speed rarely exceeds 30 mph. Some of the more sophisticated road slugs have cut out devices where the slug stops operating at higher speeds as it ceases to be useful and it is only draining useable power from the mother unit at that point. CSX just converted a number of GP30's and GP35's to road slugs (the units look like regular Geeps except for the lack of radiators and exhausts, and retain their cabs.) They are also creating Slug mothers by taking SD40-2 locomotives de-turbocharging them for slug capabilities with the recently acquired CR slugs: See CSX 2412

Although they look similar, the EMD transfer units (Cow-Calf) had prime mover diesel engines in the "B" unit(calf), with exhaust stacks and radiators". - Info provided by Doug Kroll