Try Beach Drive on a Saturday, or Sunday when it is closed. Park near Military Road and ride the section from Military to Broad Branch rd. This section is all downhill and is over a mile long. :-)
For real mountain terrain, any mountain which allows mountain biking should be OK with mountain boards. I suspect Timberline would allow them on the Salamander Trail--a trail open to MTBs and also used as an access road for staff on ATVs. If you want steeper, you'll probably have to brave some single track--there are some excellent trails just above Frederick in the Catoctins.
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