Union Pacific mystery train.

Does anyone know whet this is? some kind of maintenance train? Looks like it's designed to go in a small tunnel.
Caught this last week in Covina CA along the Metrolink San Bernardino line.

[@JonRailVideos2011-1]

The mini Loram rail grinder! Sweet catch and video! I have seen it just sit around Mission Tower in Los Angeles but haven't seen any footage till your video just now. -Jon

[@MadMaxTrac]

Thanks, now I know what it is.