EMD's MP15 series of switcher was more of a 'hybrid' yard switcher-road switcher. It looks like a normal yard switcher, but has EMD's Blomberg trucks, which is which allows it to be a road switcher. The reason for the Blomberg trucks was because it's predecessor, the SW1500, had either AAR type A switcher or Flexicoil switcher trucks, both of which have friction bearings. The FRA (Federal Railroad Administration) has banned equipment with friction bearings to be used on mainlines. Replacing the friction bearing trucks with the Blomberg trucks, which are roller bearings, allows the MP15 series switchers to be road switchers.
Here's a link about the MP15 series:
http://en.wikipedia.org/wiki/EMD_MP15DC