Once again I have my head in my hands and I draw a deep breath as I read another thread on the dear old internet where so many seem to get into such difficulties over counties!
Firstly, ignore postal addresses. Royal Mail are not in charge of counties. They were (note: WERE) part of addresses only to help post get to the correct sorting and delivery offices, especially in the days before postcodes were so widely used. The county used was that of the post town (which is why you had many examples of villages in county X but were addressed to county Y as their post town was in county Y) and the counties used sometimes did not change as counties in the real world did (most notably in 1974 and again in the mid-90s). Counties are no longer necessary in addresses - only the post town and post code are required by Royal Mail.
http://en.wikipedia.org/wiki/Postal_counties_of_the_United_Kingdom
Secondly, on "old" versus "new" - counties, boroughs, etc, have been changing boundaries and have been created and abolished for centuries. Obviously some have stood the test of time, others have come and gone, and some have returned from the dead. What makes life more complicated is that at the moment we have different systems of local government in place in each of the four parts of the UK.
Talking specifically about England, we have two types of county in use right now: actual local government counties (these come in various guises) and the geographic or "ceremonial" counties. The two actually fit together - ceremonial counties are composed of one or more local government counties... the ONLY exception being Durham/North Yorkshire which divides the borough of Stockton-on-Tees along the River Tees. Two grey areas exist regarding counties: Greater London and the Isles of Scilly - they lie outside the local government county system, and in terms of ceremonial counties Greater London has two (the City of London, and then the 32 London boroughs as the other) whilst the Isles of Scilly are part of Cornwall.
http://en.wikipedia.org/wiki/Metropolitan_and_non-metropolitan_counties_of_England
http://en.wikipedia.org/wiki/Ceremonial_counties_of_England
Now, the thing is, all sorts of places are "counties" in a purely local government sense - all metropolitan counties, all non-metropolitan counties AND unitary authorities
outside Berkshire (don't ask), Greater London, the Isles of Scilly and the metropolitan counties. Because of this, increasingly the geographic/ceremonial counties are used. They cover all of England and they more closely match the historic/traditional counties many relate to.
I'll give an example of all the above regarding the question of the thread and my county of Shropshire. If we're talking in terms of local government counties, then Telford & Wrekin (a unitary district - it was made a county in local government terms in 1998 to become 'unitary') cuts in between Shifnal and Shrewsbury on the Wolverhampton-Shrewsbury line. In geographic/ceremonial terms, this doesn't happen as Telford & Wrekin is part of Shropshire. This is of course what the vast majority of people in Shropshire consider correct - the idea that Telford & Wrekin is either a county in its own right or outside Shropshire is absurd. The only (presently functioning) railway line then which goes in and out of Shropshire is the HOWL, which enters Herefordshire once or twice (very briefly) between Hopton Heath and Bucknell.
Apologies for the lecture! But it does frustrate me how people just don't know (or think they know, but don't) about counties. And the endless arguments... especially involving the bloody Royal Mail..!