# 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

Ticket
:   A single tracked trip. These are imported from the GTFS (or couple be created
    manually via the web interface), and are independent from it, i.e. they are
    saved in tracktrain's data base and won't change with subsequent GTFS updates,
    but would have to be deleted and reimported instead.
    
    This prevents tracktrain from ending up with invalid data if a trip's id
    or stops change retroactively.

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")

(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.

Seconds (on a given 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).

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.

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.

(Train-)Anchor
:   An "anchored" point of position of a train along a trip; a snapshot of its
    delay and state at a known position. These are generated from train pings,
    and are the basis for extrapolating future delays for passanger information.

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.