I didn't see a single full-size car, boat, or airplane, and the only full-size train is a single Hiroshima street car parked outside. Aside from a collection of motorcycles and scooters and a few interpretive pieces (e.g. engine cutaways), it's all models. And while 1/80 scale models are a great way to see the difference in size between a Boeing 737 and an Airbus 340, they don't impress the way real artifacts might. Very little of the interpretive material is in English.
The museum is right next to the Astram line's depot, and it feels like it may have been built either to motivate people to ride the line (at 390 Yen each way from downtown) or as a condition of the city's granting the Hiroshima Rapid Transit company its franchise. But it's a terrible museum. The third and fourth floors are taken up by a "future transportation" model layout. It's impressive for its scale and the variety of moving parts, but it's grotesquely bland: simple flat paint with no landscaping, some interpretive video systems (all in Japanese), and enough theatrical lighting implements to run four simultaneous crappy college plays. But you won't learn anything from it. If they had instead put in interpretive material about the Astram line, perhaps with a window into the shop floor, it might be more interesting.
This is not worth spending a 25 minute (each way) train trip and 1280 yen on. (500 admission, plus 390 train fare each way.)
I'm giving it a second dot because the sheer number of (often plastic) plane, boat, and train models IS impressive. Several thousand, I'd guess, mostly at 1/100 to 1/80 scale. But the scales aren't consistent, so comparisons are sometimes tricky. Behind the museum is a bicycle-riding track that seemed to be popular with the kids: they hire a wide variety of human-powered conveyances.
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