Increasing the capacitance requires larger and more expensive capacitors but increasing the supply voltage is relatively inexpensive. Be warned though, before doing this make sure all the components are suitably rated for the increased voltage. Do not operate any components beyond their recommended safe tolerances. Doubling the voltage results in four times the energy in the capacitor.
Usually the only power supply available on model railway layouts is either 12 volts DC or 16 volts AC. For this voltage booster we use a modified version of the +12 and -12 volt power supply unit as used previously. To be absolutely clear, the voltage difference between +12 and -12 is of course 24 volts.
Is this higher voltage safe in a model railway environment? Indeed it is. Because the transistor switches off very quickly the solenoids cannot be overloaded. The higher voltage is quite safe as the energy is delivered for a very short time only. Touching the terminals will cause no real sensation unless your hands and body are very wet and you are standing naked in a half filled bath. You wouldn't want to be doing that, now would you?