The lack of a train describer would not be an impediment to showing the tram line to Midland in the NR Planning systems. After all, it shows the Supertram network as far as Cathedral.Maybe because Metro has something similar to conventional railway signalling, and presumably there's a train describer to interface to that looks similar to the ones on Network Rail. Sheffield is a tramway and doesn't have block sections or train describer berths in the conventional sense. I think there's something that keeps track of where a tram is between the transponder loops that are provided at various places, but there can be more than one tram between each pair of loops so it's probably not straighforward to make this look like a traditional train describer. Also it's probably been in use for the 30 years since Supertram opened, and there may no longer be anyone around who understands how it works.
This may be down to how Metro trains were planned to be shown in the system. It's also possible that Metro was added at a time when NR had people willing to put in a bit more detail than strictly needed. It's noticeable that the Manchester Metrolink connections are generally not shown beyond the NR/Metrolink ownership boundaries in TPS.Metro on the Durham Coast, the industry systems have every possible terminating point on them including those without booked workings such as St James.
I'm fairly sure the Bakerloo line isn't shown south of Queen's Park in NR planning systems. North of Queen's Park is shown in full because it's shared with the DC Line.And I think this might also apply for the Bakerloo line too.
I suspect NR were asked to put in sufficient infrastructure to show the intended service pattern, and nobody considered/asked/had the time to put in other locations. Drawing the infrastructure in the planning system, especially in a tight area like Sheffield, can be very fiddly. If trams are only going to terminate at Midland once in a blue moon, NR may well ask what is the point in allocating the resources to input somebody else's network. I think they should have included Midland when they drew it, but I can see why they didn't.So not sure why it’s missing for Sheffield.