proposal: trip_short_name

[Joe Hughes]

Summary:
Add a trip_short_name field to trips.txt to better identify services
(like commuter rail) which present a meaningful trip name to riders.

Motivation:
Some public transit services, particularly commuter rail lines, expose
a trip name to riders in schedules (see a Caltrain example here in the
"Train No." row: http://www.caltrain.org/timetable.html#weekday-northbound
) and on in-station platform labels. In order to help orient riders,
it would be useful to represent those trip short names in trip planner
interfaces.

Proposal:
A new trip_short_name field would be added to trips.txt, with a
description as follows:
--------------------------------
trip_short_name (optional):
For individual trips that have a short rider-facing name in schedules
and station signage (for instance, commuter rail train numbers), this
field provides that name. If riders do not commonly identify trips by
name, please leave this field blank.
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Joe Hughes
Google Transit