Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Rail
MG_SPLINE_RAILParameter: Target
MG_SPLINE_RAIL_TARGETParameter: Scale
MG_SPLINE_RAIL_SCALEParameter: Distribution
MG_SPLINE_MODEParameter: Count
MG_SPLINE_COUNTParameter: Step
MG_SPLINE_STEPParameter: Per Segment
MG_SPLINE_PERSEGParameter: Smooth Rotation
MG_SPLINE_SMOOTHParameter: Offset
MG_SPLINE_OFFSETParameter: Offset Variation
MG_SPLINE_OFFSETVARParameter: Start
MG_SPLINE_STARTParameter: End
MG_SPLINE_ENDParameter: Loop
MG_SPLINE_LOOPParameter: Rate
MG_SPLINE_RATEParameter: Rate Variation
MG_SPLINE_RATEVARParameter: Random Seed
MG_SPLINE_SEEDParameter: Volume Spread
MG_SPLINE_VOLUME_SPREAD