  Time-reversal symmetry of most conservative forces constrains the properties
of linear transport in most physical systems. Here, I study the efficiency of
energy transfer in oscillator networks where time-reversal symmetry is broken
locally by Lorentz-force-like couplings. Despite their linearity, such networks
can exhibit mono-directional transport and allow to isolate energy transfer in
subsystems. New mechanisms and general rules for mono-directional transport are
discussed. It is shown that the efficiency at maximum power can exceed $1/2$
and may even approach the upper bound of unity.
