A W-Element model is a simplified model of one or many transmission lines used in circuit simulation. It is basically an RLGC matrix with a line length included, thus it contains all of the transmission and material parameters such as skin-effect/roughness losses, dielectric losses, characteristic impedance and mutual coupling. It is typically created with a field solver, and then used within a schematic-based circuit simulation environment.
The RLGC telegrapher's equations are valid for TEM transmission lines. Although microstrip lines are not purely TEM, the error is minimal and RLGC models are still used. This is referred to as quasi-TEM.
W-element models can be used in many simulation tools such as Cadence Sigrity, Ansys Electronics Desktop and Keysight ADS. They can be generated in these tools or with the open-source TNT/MMTL Field Solver (though TNT is limited to lossless lines).
 D.B. Kuznetsov, E. Schutt-Aine: Optimal transient simulation of transmission lines (https://ieeexplore.ieee.org/document/486433)
 H-Spice SI User Guide, accessed at http://www2.ece.rochester.edu/courses/ECE222/hspice/hspice_si.pdf
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