I concur with others to tailor the itinerary to suit your needs on Trainsplit (which will also figure out of it's cheaper to split or not)
Realtime Trains will tell you. If it's not reservable then it won't say anything.
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True, and so will the link available from Trainsplit to the relevant train on Timetablehistory.com (bottom left rather than top right)
I.e. Can I get an earlier train from Waterloo in order to “break” my journey to have supper in Kings Cross before I board for Newcastle.
Just to clarify, break of journey is a defined term which means leaving railway premises (other than for the purpose of interchanging between stations).
You are not allowed to break your journey intermediately on a through Advance fare, however if you have supper within King's Cross station (e.g. at any of the many outlets within the station, such as the
Parcel Yard etc.) that is absolutely fine as this
does not constitute a break of journey; it's also fine to do so at St Pancras which increases the range dramatically (e.g.
Carluccios is a good place for a proper sit-down meal).