It's fairly clear that the OP is talking about Parliamentary services, as discussed in tonight's Trainspotting Live which used both terms.
Not everyone has seen that
Without having watched it, I suspect they used both terms because the aforementioned service is both infrequent (though it's actually more frequent than a
parliamentary service which is arguably only once a week)
and very lightly used.
If you want to invent a new meaning like "almost unused trains" then let's have a new name.
The term "Ghost train" can mean anything and is used by various preserved railways for services at Halloween, though perhaps not often as graphically as proposed in the
Ghost train thread by Kris.
I don't think I've
ever heard anyone refer to a "parliamentary" (once per week) or infrequent service as a "ghost train" in general conversation.
Some people use the term to refer to a train that does not appear on open data sites e.g.
Ghost Train (posted by Deepgreen) or in timetables e.g.
1540 Glasgow to London not advertised at Preston ('Ghost train') &
Ghost Train (posted by Hoover) or shown in one timetable but not another e.g.
19:20 Virgin Euston-Manchester Ghost Train
It appears to be a term that can mean almost anything anyone wants it to mean.
The Scarborough to London service discussed here
http://www.railforums.co.uk/showthread.php?t=132654
would fit in that category
Not when I've caught it
, as would quite a lot of other very early morning or late night trains! In their last years the summer Fridays-only overnight Manchester to Paignton (?) service would have qualified: there were only about a dozen on it when I used it with the family, likewise an overnight Manchester (?) to Brighton service.
If that is the purpose of the thread, it would have some merit, but if the purpose is to list infrequent services, then I agree with others that it's been done to death lately
(and with more appropriately named thread titles )