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# Glossary
This is meant to give a rough overview of (train-related) terms used in
this code; both for others as a reference and for me so I can remember
to use them in a (somewhat) consistent way, since they are somewhat
arbitrary. I've tried to remain at least broadly close to the
terminology used by GTFS.
## Terms
(Calendar-)Date / Day
: A single, unique day (e.g. 1970-01-01). Usually used to indicate if a
*trip* is running on that day or not.
Time (of Day)
: Time on a given day, given in seconds (though often displayed as
minutes) since midnight. If a trip crosses midnight it is treated as if
it took place entirely on the previous day, and times simply count up
beyond the total number of seconds in a day (note that that's a
timezone-series dependent number).
Trip (don't confuse with Train)
: Used as in GTFS: a trip is a defined sequence of *stops*, referred to by
a number (called its trip ID, e.g. IC 94). Usually runs on multiple
days. Always has an associated *shape*.
(might match your intuition for "train line")
Stop
: A *station* with associated arrival/departure *time*.
Station
: A train station. Tracktrain refers to each by an ID, and hopefully knows
its geolocation.
Shape
: A sequence of geolocations describing a line between stations,
describing the physical railway along which trains travel.
Train (don't confuse with Trip)
: A single instance of a *trip* on a concrete *date*. Tracktrain mostly
concerns itself with keeping track of those; the rest is just additional
stuff.
Vehicle
: An actual, physical vehicle, which might act as the *train* going along
a *trip* on a certain *date*.
For now tracktrain doesn't really care about them (but if it's curious
it might yet learn about them!)
Announcement
: The thing that GTFS calls "Service Alert" --- a text message giving
human-readable information about some *train*.
(Train-)Ping
: A single packet of data sent from a train's *OBU*. Might arrive in some
arbitrary order.
Control Room
: The "admin interface" of tracktrain, which is not meant to be used by
on-board staff.
On-Board Unit (OBU)
: A thing on a vehicle which does geolocation tracking and yells at
tracktrain about it.
If we ever run into potential confusion regarding this term we're
probably way too professional to actually use tracktrain for anything.
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