The main downside of double-decker train cars is the time it takes passengers to to board them. And, since this is one of the main factors limiting metro frequencies and thus capacity, they’re not that suitable for subways. To maximize metro capacity, you want long trains with many doors and very high frequency.
Double-decker cars are much more suitable for lower-frequency service (S-Bahn, regional, long-distance,…) where they’re also commonly used.
Of course, you could still use double-decker cars in a metro (and maybe some places do), it’s just suboptimal.
The main downside of double-decker train cars is the time it takes passengers to to board them. And, since this is one of the main factors limiting metro frequencies and thus capacity, they’re not that suitable for subways. To maximize metro capacity, you want long trains with many doors and very high frequency.
Double-decker cars are much more suitable for lower-frequency service (S-Bahn, regional, long-distance,…) where they’re also commonly used.
Of course, you could still use double-decker cars in a metro (and maybe some places do), it’s just suboptimal.
Two level platform? Then you’re actually boarding double the number of passengers, could be useful in very busy stations.