Hmm, this Q only comes up about once a week! Problem is,
most people answer it incorrectly. SMEE is an "air brake
schedule designation" made up by WABCO (please don't ask
what WABCO means!) A schedule is basically a parts list.
Schedule codes follow a certain pattern.
In this case:
S -> Straight Air, as opposed to A (Automatic). Straight air
systems are characterized by a trainline in which
air pressure is proportional to the degree of braking
effort desired. Automatic systems use a trainline
in which the braking effort call is proportional to
the difference between the trainline pressure and the
reference pressure.
M -> Motor car equipment. As opposed to "T", trailer car. "M"
schedules generally include compressor, reservoir and
motorman/engineer's brake valve, in addition to the
equipment that comes with "T" schedules (cylinder(s),
control/triple/universal valve, aux reservoir (if applicable))
E -> Emergency. The addition of an emergency trainline pipe makes
the straight air system failsafe. "A" (Automatic) schedule
systems accomplish this with a single pipe. Schedule
"SME" was a common streetcar apparatus where trailers
(schedule STE) were hooked on for rush hour capacity.
The emergency pipe is also called the Brake Pipe.
E -> Electropneumatic. This means that electric trainline circuits
are used to speed the propagation of the brake request
signal through the train. On NYCT equipment, these trainlines
are called "A" and "R" wires. There is also an "EMV" wire
which provides an electric emergency application.
Without the last E, SME would be too unresponsive for the
500-600' train lengths that NYCT operates.