How well does the locomotive run by itself? I'm no DCC expert, but depending on the decoder, there may be a CV to set for maximum speed.
Also, the smoke unit and incandescent lamps in the passenger cars may be drawing a lot of the current. The power supply output for the DCS50 is 2.5 amps. You may need a more powerful power supply. If the Locomotive is OK, try adding one car at a time to see if that affects speed.
While DC, I know that seven AM Budd's and two AM E8's is about the maximum that my 3.0 amp DC power supply can handle.
Rusty