  We theoretically study transport properties of voltage-biased one-dimensional
superconductor--normal metal--superconductor tunnel junctions with arbitrary
junction transparency where the superconductors can have trivial or nontrivial
topology. Motivated by recent experimental efforts on Majorana properties of
superconductor-semiconductor hybrid systems, we consider two explicit models
for topological superconductors: (i) spinful p-wave, and (ii) spin-split
spin-orbit-coupled s-wave. We provide a comprehensive analysis of the
zero-temperature dc current $I$ and differential conductance $dI/dV$ of
voltage-biased junctions with or without Majorana zero modes (MZMs). The
presence of an MZM necessarily gives rise to two tunneling conductance peaks at
voltages $eV = \pm \Delta_{\mathrm{lead}}$, i.e., the voltage at which the
superconducting gap edge of the lead aligns with the MZM. We find that the MZM
conductance peak probed by a superconducting lead $without$ a BCS singularity
has a non-universal value which decreases with decreasing junction
transparency. This is in contrast to the MZM tunneling conductance measured by
a superconducting lead $with$ a BCS singularity, where the conductance peak in
the tunneling limit takes the quantized value $G_M = (4-\pi)2e^2/h$ independent
of the junction transparency. We also discuss the "subharmonic gap structure",
a consequence of multiple Andreev reflections, in the presence and absence of
MZMs. Finally, we show that for finite-energy Andreev bound states (ABSs), the
conductance peaks shift away from the gap bias voltage $eV = \pm
\Delta_{\mathrm{lead}}$ to a larger value set by the ABSs energy. Our work
should have important implications for the extensive current experimental
efforts toward creating topological superconductivity and MZMs in semiconductor
nanowires proximity coupled to ordinary s-wave superconductors.
