A few things:
Double deck does not mean double capacity, as you need space for stairs and probably can't have standing in the upper saloon.
DART OHLE cables are at about 4.5m above rail under bridges, which amkes it a bit tight for double deck trains. At level crossings DART and Laus cables are raised to about 5m above road
DART Underground will be 50mm too low for double deck trains!
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