Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Z Spline Left
SPLINERAILOBJECT_SPLINE1
Parameter: Z Spline Right
SPLINERAILOBJECT_SPLINE2
Parameter: X Spline Top
SPLINERAILOBJECT_SPLINE3
Parameter: X Spline Bottom
SPLINERAILOBJECT_SPLINE4
Parameter: Reference
SPLINERAILOBJECT_REFERENCE
Parameter: Mode
SPLINERAILOBJECT_MODE
Parameter: Size
SPLINERAILOBJECT_SIZE
Parameter: Scale Before Start
SPLINERAILOBJECT_SCALEBEFORESTART
Parameter: Scale After End
SPLINERAILOBJECT_SCALEAFTEREND