It turns out that Circuit Smasher is...

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...

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...Spike Witwicky. Who must be in his forties now, so he's a bit like one of those embarrassing dads who mow the lawn with no shirt on. Also, Rack'n'Ruin and Roadbuster are back... at least until Spike gets to them, it seems. Also Whirl, Springer and... Chromedome? Who's that in the bottom left?