The service and emergency (figures from Wikipedia) braking speed
of a SMRT Series 0 (C151) are 2.8 km/h/s and 4.68 km/h/s, so there
is quite a big difference, most likely the jerk from the transition
of max normal brake to emergency brake will make people fall
down.
If there is a constant 'pssh-pssh' sound, likely its from the
re-adhesion action, akin to the car ABS (anti-lock braking), the
air brake applying and releasing to prevent wheel skidding
especially on wet rails.
It would be due to the design that there is little re-adhesion
entirely based on regenerative brakes on any of our trains, as the
air brakes are also in charge of the normal braking force... A
search on videos online will get the meaning.
Notes: Air brake means using the air reservoir for braking
purpose, a padded brake is used in this case. The other type would
be a disc brake. This is a form of dynamic braking (spent energy
and cannot get back).
Regenerative braking (spent energy and can be reused) means
converting kinetic energy into electrical and feeding it back into
the system at the same time. A long explanation to go in-depth, but
in short it saves energy by giving back energy to the electric
supply during braking to be spent on another train's acceleration
process. This process also enables the train to slow down, and
depending on strength of the regeneration, dynamic (air brakes)
braking is also necessary to be used together.
There are some trains in the World which has regenerative
braking all the way to 0 km/h, the JR East E231 is one. Dynamic
braking system is also carried but reserved for emergency and/or
regenerative loss.
Our trains have regenerative cut at 6 km/h (thanks to E5さん for
the info) and dynamic braking is used thereafter.