Express can work if there is a triple tracked section, and you
overtake then.
A third track could be used to run express services exclusively
in the peak direction, and aid in redundancy. However, passengers
must expect that the express service will not be possible should
there be a track disruption, and not plan their journeys with that
in mind.
One way Japan might do to its Chūō Rapid Line after its
elevation from at-grade tracks is to built two parallel underground
tracks to run an express service.