Rail link to BFS
It's not realistic to envisage there ever being a rail link to BFS.
Aside from the chicken & egg situation of PAX numbers not being sufficient to justify the expenditure on such a link...
How do you help boost numbers? By making access easier and competing with the ease of access offered by BHD.
The problem with a rail-link spur to the line at RWY 07 threshold is the track infrastructure onwards from there to Belfast.
IIRC NIR mothballed that line in 2003 - the resulting upgrade work to bring it back into service and the additional cost of dualling the track (from current single track) or provision of sufficient passing loops to allow a reasonable freq or service would be prohibitively expensive.
To go the other way via Antrim would require a spur loop to be constructed south of the Belmont rd to avoid having to switch ends in Antrim station.
A much easier solution would be a 7 mile motorway spur from the M2