Whts the issue when its not disturbing any traffic and been conducted @ midnight???
Dunno which kinda work engine u talking about & how they monitor track without getting attached with any other train (there exist some four wheel mechanised/non-mechanised unit to monitor track, which mayb of no use in heavily congested section)!!
Further, Track Monitoring & Speed Monitoring do have difference & the complete Unit will cover track condition, speed impact (through analysis & reporting and based over which, certain defects over points on section would be taken care of!!