Clive,
We have a track cleaning loco on the B&BCRR and have been using it for coming on 6yrs without any major problems, I have had on occasion the same problem of the abrasive wheels out gripping the drive wheels but generally adjustment of the speed of the abrasive wheels clears the fault (In case you don't know that is the little black knob on the top) another way is to send it around the track in the opposite direction, this can also assist. We have 3% grades on our RR and it copes with these quite satisfactory but as has been said DO ensure the track is dry.
When running I generally have a cleaning truck in the consist and clean / scrape the crud off about every 45 mins or so, this is more important when running loco's that don't have pickup skates as well.
hope this helps.
regards.