You get that often when the session is set to default to DCC mode and you have Raildriver with the reverser not exactly centered, it reads the Raildriver reverser as DCC forward and by the time it stops loading scenery and responds to your CAB mode selection the loco is already moving. Simplest cure is to edit the session to default to CAB mode and uncheck the box to load the quick drive window at start.
If the locos are driven by AI drivers, they will be in motion before the game has fully loaded into Driver.
What I do is put in a wait command for about 30 secs. This gives the program a chance to load and settle in, and I noticed too that everything runs much better this way.
You get that often when the session is set to default to DCC mode and you have Raildriver with the reverser not exactly centered, it reads the Raildriver reverser as DCC forward and by the time it stops loading scenery and responds to your CAB mode selection the loco is already moving. Simplest cure is to edit the session to default to CAB mode and uncheck the box to load the quick drive window at start.
This was my problem, I was unaware that the DCC control read the RailDriver reverser as a throttle cue. This will come in handy where sessions only behave properly when in DCC mode, I can use my RailDriver reverser rather than the mouse on the little controller at the bottom of the screen.