Circuit simulation shows gain with only passive components. Why?
How is it possible that any voltage in the simulated circuit is larger than the input?


Voltage gain is not the same as power gain, and this is exactly what happens at resonance. 

Your circuit has an ac source that could potentially continuously add power to the circuit. But it has no lossy elements (resistors or resistive parastics), so there is no where for power to to be removed from the circuit. In fact, of course, this circuit has only reactive impedance so no real power is ever actually delivered by the source.

In fact you can have the voltage at an intermediate point in the circuit go to infinity even with a much simpler circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

At resonance, the input impedance of the series LC combination goes to zero. So the current delivered by the source goes to infinity. But the impedance across C1 is not zero, so therefore the voltage at the intermediate node (where L1 and C1 are connected) goes to \$-\infty\$ at resonance. 

Of course in a real circuit there would be some parasitic loss somewhere and neither could the voltage source deliver infinite current, so the actual voltages would all remain finite. But in the ideal circuit approximation, you expect the possibility of infinite voltages (or currents) at resonant frequencies.