  Nonlinear, relativistic longitudinal waves with sub-luminal phase velocity
$v_p$ are the basis of plasma-based electron accelerators. For such
application, key properties of the wave are the maximum or ``wave breaking''
amplitude and the corresponding energy gained by electrons trapped in the wave
field. Here we show that these properties and the general waveform are obtained
with little mathematical complexity by using a Lorentz transformation to a
frame co-moving at velocity $v_p$. The transformation reduces the problem to a
second-order ordinary differential equation as originally found by Chian
[Plasma Phys. \textbf{21}, 509 (1979)] so that the analysis can exploit the
analogy with the mechanical motion of a particle in a potential well. This
approach seems particularly suitable for a compact, tutorial introduction to
plasma-based electron accelerators with little mathematical complexity.
