The first Thunderbolt is in Hatley park in Rollingwood, an Austin neighborhood just west of Mopac right after you cross the Colorado river, at 4821 Rollingwood Drive. I didn't realize it at first but the Rollingwood police department is tucked away in the trees about 50 yards from the siren, which is why I assume they put it there. But basically you'll drive west down Rollingwood Drive and eventually you'll see it on your right when you get to the baseball fields. This is kinda what you'll see from the street:
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Here are a couple other pictures I took of it:
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Here's a couple angles highlighting the missing blower:
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If you were to visit one of these three Thunderbolts, this would be the one I suggest you opt for. It's in a beautiful location, and as you'll soon understand, long and aways the most easily accessible.
The second Thunderbolt I spotted over the hills was this one at 2115 Rabb Road near Barton Springs. It's literally in someone's backyard (I'm a little jealous) which makes it hard to physically approach. The lot behind the property with the Thunderbolt used to be vacant so I could walk right up to the fence and a couple feet away from the siren, however the lot has since been developed and a new house is blocking the approach. So basically you're stuck with a street view, unless you want to sneak around the side of the house (which I actually did once; shh, don't tell anyone) or ring a doorbell. But, here are a couple photos of it. I used to have more but for whatever reason some have been lost.
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My third and final Thunderbolt was found purely by dumb luck. It's located at the Camp Mabry Texas National Guard, right on the street. After looking at several fire and police stations around Austin, I thought, "hmm, maybe the army base." And lo and behold, right on the street was a Thunderbolt. It's located at 2450 W 35th street, right off Mopac. Again, this one isn't the easiest to get up to, as parking is limited and it's on a busy street, but I've still managed to walk up to it a few times and check it out. Here she is:
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Hope the links work this time!