  We present an extensive spectroscopic follow-up campaign of 29 strong lensing
(SL) selected galaxy clusters discovered primarily in the Second Red-Sequence
Cluster Survey (RCS-2). Our spectroscopic analysis yields redshifts for 52
gravitational arcs present in the core of our galaxy clusters, which correspond
to 35 distinct background sources that are clearly distorted by the
gravitational potential of these clusters. These lensed galaxies span a wide
redshift range of $0.8 \le z \le 2.9$, with a median redshift of $z_s = 1.8 \pm
0.1 $. We also measure reliable redshifts for 1004 cluster members, allowing us
to obtain robust velocity dispersion measurements for 23 of these clusters,
which we then use to determine their dynamical masses by using a
simulation-based $\sigma_{DM} - M_{200}$ scaling relation. The redshift and
mass ranges covered by our SL sample are $0.22 \le z \le 1.01$ and $5
\times10^{13} \le M_{200}/h^{-1}_{70}M_{\odot} \le 1.9\times10^{15}$,
respectively. We analyze and quantify some possible effects that might bias our
mass estimates, such as the presence of substructure, the region where cluster
members are selected for spectroscopic follow-up, the final number of confirmed
members, and line-of-sight effects. We find that 10 clusters of our sample with
$N_{mem} \gtrsim 20$ show signs of dynamical substructure. However, the
velocity data of only one system is inconsistent with a uni-modal distribution.
We therefore assume that the substructures are only marginal and not of
comparable size to the clusters themselves. Consequently, our velocity
dispersion and mass estimates can be used as priors for SL mass reconstruction
studies and also represent an important step toward a better understanding of
the properties of the SL galaxy cluster population.
