1. A method for operating a data processing system to simulate a circuit comprising a plurality of circuit devices connected by conductors, said method comprising: providing a layout description of said circuit in which said circuit devices are connected by interconnects, each interconnect being associated with a line definition comprising a physical description of an interconnect between two of said circuit devices and a simulation model to be used in simulating said interconnect during simulations of said circuit, said line definition being user selectable from a list of possible line definitions; causing a netlist to be generated from said layout description, wherein one of said interconnects is replaced by a plurality of transmission line devices, each transmission line device being associated with said simulation model included in said line definition; and causing said circuit to be simulated utilizing said netlist.