It doesn't double the capacity. It's more a 30% increase. And, double-deckers are actually old. They date from the 19th century. In 1855, the Chemin de fer de l'Ouest introduced double-deckers on suburban lines of Paris.
I know that train model, we had them here too; they're pretty heavy and expensive compared to the lighter rail models that many companies are buying now. It has quite a heavy frame compared to newer models.
I also suspect most tesla workers don't come by train anyway, so they don't need that much capacity.
I like them for nostalgia reasons, but definitely not for quality reasons. Though I must say, the rail service has refreshed them a few years back and they're not as bad as they used to be, though it does feel like they're the model that breaks down the most whenever there's some sort of extreme weather
The biggest problem is acceleration, though, which is still an issue for lines with many stops like the ones you see here. I imagine they'd do a lot better in countries that have lines where trains can go at least half an hour without stopping anywhere.
Maybe we'll see the return of double deckers once Tesla makes its first double decker car (I'm thinking "terrible electric RV with a tiny sleeping area on top") so they're cool again.