It is a permanent path for a departmental service, regularly used for stuff such as leaf fall treatment in the autumn. But the paths are in the system all year round, which makes it easier for them to run any other form of maintenance train, e.g. weed killing, or a visual inspection.
The area's departmental MPVs (those yellow ended freight DMU things) are operated by SWT drivers (which is why they show as SW services) but the trains are based at Effingham Jn and some other places such as Eastleigh.
If you follow this link you'll see a couple of similar services doing other loops round the SWT network:
http://www.realtimetrains.co.uk/search/advanced/XEJ/2015/05/19/1612
I think having the TOC staff drive them around is a particular feature of our local setup with the 'SWT/NR Alliance', nationally it is more usual for departmental MPVs to be operated by one of the freight companies.