  We present results on the compact steep-spectrum quasar 3C 48 from
observations with the VLBA, MERLIN and EVN at multiple radio frequencies. In
the 1.5-GHz VLBI images, the radio jet is characterized by a series of bright
knots. The active nucleus is embedded in the southernmost VLBI component A,
which is further resolved into two sub-components A1 and A2 at 4.8 and 8.3 GHz.
A1 shows a flat spectrum and A2 shows a steep spectrum. The most strongly
polarized VLBI components are located at component C $\sim$0.25 arcsec north of
the core. The polarization angles at C show gradual changes across the jet
width at all observed frequencies, indicative of a gradient in the
emission-weighted intrinsic polarization angle across the jet and possibly a
systematic gradient in the rotation measure; moreover, the percentage of
polarization increases near the curvature at C, likely consistent with the
presence of a local jet-ISM interaction and/or changing magnetic-field
directions. The hot spot B shows a higher rotation measure, and has no detected
proper motion. These facts provide some evidence for a stationary shock in the
vicinity of B. Comparison of the present VLBI observations with those made 8.43
years ago suggests a proper motion of $\beta_{app}=3.7\pm0.4 c$ for A2 to the
north. The apparent superluminal motion suggests that the relativistic jet
plasma moves at a velocity of $\gtrsim0.96 c$ if the jet is viewed at an
inclination angle less than $20\degr$. A simple precessing jet model and a
hydrodynamical isothermal jet model with helical-mode Kelvin-Helmholtz
instabilities are used to fit the oscillatory jet trajectory of 3C 48 defined
by the bright knots.
