I'd say that it could be possible for a S7 to stand in for a S8, but not the other way around given the extra length of an S8.
While for the shared section of track the platforms are long enough, I presume they're not on the rest of the line - or else they'd all be S8s.
While for the shared section of track the platforms are long enough, I presume they're not on the rest of the line - or else they'd all be S8s.