In electric or mixed electric-mechanic systems, the distinction between potential and kinetic energy is not as clear as in purely mechanical systems. A solution for the motion of an inductively loaded rail generator is presented. In this case, the magnetic field energy (1/2)Li[superscript 2] can be written "formally" in terms of a potential energy, while physically it is something different. The analogy between mechanical and electric oscillators has limits, and harmonic oscillators can operate without potential energy, both of which must be pointed out by physics teachers.