Douglasville is a small city west of Atlanta near I-20, and is not MARTA accessible. There are regional buses operated by GRTA Xpress that travel to the area Monday-Friday on a very limited schedule:
Take any train heading downtown to the Five Points station and take the Forsyth Street exit out of the station to catch GRTA Xpress 461/462. The bus departs only Monday-Friday between 4:30 and 6:30 p.m.
Take a red or gold line train to the Civic Center station for GRTA Xpress 460. The bus departs only Monday-Friday between 3 p.m. and 6 p.m.
The other option is to use MARTA and a taxi. If coming from the blue or green rail line, board a H.E. Holmes Westbound train and take it to the H.E. Holmes station and arrange for a taxi there at 70 Hamilton E. Holmes Drive (about 17 miles from Douglasville). If you are coming from the airport, Midtown, Buckhead or otherwise on the red or gold lines, take the train to the Five Points station and transfer to the Westbound train platform and wait for the H.E. Holmes train there.
While MARTA does not extend into Gwinnett, you can connect with Gwinnett County Transit at the Doraville MARTA station at the end of the gold rail line. GCT Route 10 departs from the station and travels north on Buford Highway through Norcross, ending at the Gwinnett Transfer Center/Gwinnett Place Mall. The transfer center allows connections between all GCT routes.
Transfers to/from MARTA are free — the Breeze Card can be used as payment on all area transit systems. To save time, request an Uber or Lyft car at the Doraville MARTA station or take one of the waiting taxis to your final destination.