Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Z Spline Left
SPLINERAILOBJECT_SPLINE1Parameter: Z Spline Right
SPLINERAILOBJECT_SPLINE2Parameter: X Spline Top
SPLINERAILOBJECT_SPLINE3Parameter: X Spline Bottom
SPLINERAILOBJECT_SPLINE4Parameter: Reference
SPLINERAILOBJECT_REFERENCEParameter: Mode
SPLINERAILOBJECT_MODEParameter: Size
SPLINERAILOBJECT_SIZEParameter: Scale Before Start
SPLINERAILOBJECT_SCALEBEFORESTARTParameter: Scale After End
SPLINERAILOBJECT_SCALEAFTEREND