GazK wrote:

Don't forget the loss of capacity due to increased dwell times loading
a DD train...

Dwell time issue only become significant if the train a significant
percentage of the passenger carrying capacity of the DD train
boards/alights at every stop on the route.

In most systems that doesn't happen, with large boarding/alighting
flows only occurring at a relatively small number of stops along the
route.

Bill Bolton
Sydney, Australia