  Determination of cluster masses is a fundamental tool for cosmology.
Comparing mass estimates obtained by different probes allows to understand
possible systematic uncertainties. The cluster Abell 315 is an interesting test
case, since it has been claimed to be underluminous in X-ray for its mass
(determined via kinematics and weak lensing). We have undertaken new
spectroscopic observations with the aim of improving the cluster mass estimate,
using the distribution of galaxies in projected phase space. We identified
cluster members in our new spectroscopic sample. We estimated the cluster mass
from the projected phase-space distribution of cluster members using the
MAMPOSSt method. In doing this estimate we took into account the presence of
substructures that we were able to identify. We identify several cluster
substructures. The main two have an overlapping spatial distribution,
suggesting a (past or ongoing) collision along the line-of-sight. After
accounting for the presence of substructures, the mass estimate of Abell 315
from kinematics is reduced by a factor 4, down to M200=0.8 (-0.4,+0.6) x 10^14
Msun. We also find evidence that the cluster mass concentration is unusually
low, c200=r200/r-2 <~ 1. Using our new estimate of c200 we revise the weak
lensing mass estimate down to M200=1.8 (-0.9,+1.7) x 10^14 Msun. Our new mass
estimates are in agreement with that derived from the cluster X-ray luminosity
via a scaling relation, M200=0.9+-0.2 x 10^14 Msun. Abell 315 no longer belongs
to the class of X-ray underluminous clusters. Its mass estimate was inflated by
the presence of an undetected subcluster in collision with the main cluster.
Whether the presence of undetected line-of-sight structures can be a general
explanation for all X-ray underluminous clusters remains to be explored using a
statistically significant sample.
