The trains generally originate in Benning Yard. There is no set schedule, although activity has picked up with some additional traffic in recent months.
In addition to coal going to the power plants at Chalk Point and Morgantown, there is also gypsum and limestone traffic. There is a (usually) 3 times a week stone train that runs to a facility just north of La Plata that generally operates on a Monday-Wednesday-Friday schedule. These movements are usually at night but with the long daylight hours as we enter summer there is a chance you may catch it in daylight.
Radio frequencies on the line are 161.07 and 161.34.
A couple of good places to catch any activity are in Bowie at the railroad museum which is adjacent to the wye for the Pope's Creek sub or the wye in Brandywine where the Herbert sub going to Chalk Point branches off from the Pope's Creek sub.