I find it odd that they'd even bother to run a siren on gasoline. I can see that it might have been cheaper in the 50s, 60s and 70s but Gas prices have skyrocketed from this BP mess and Propane is more of the same. I'd just stick with 240 volts of sheer 3 phase electricity. I have had my encounters, with 120 volts, not 240... yet.
But Running a siren such as the Thunderbolt, Let alone the Mobil Directo on Gas would be unbelivably expensive in these times of economy. I can see where they got the idea of running a siren on Gas, since electricity was "Dangerous" back then. But hey, If it was a good alternative to electricity, I can see that it may be cheaper, but I also see gas as more of a "Fire Hazard" than electricity.