The test car could be placed within the consist, or trailing it; it does not monitor track conditions, except peripherally, but rather acts as an "electronic dynamometer car". Todays' computer controlled locomotives actually do a lot of that data gathering and computation themselves; the test car would just tap into that, but might also monitor some extra test points that would hard wired from the test loco to the car; hence, it would probably be next to the loco being tested.
You could run some very fine ga thread, using 1/48 scale duct tape, from various points [ hood doors, etc ] on the loco to the car to simulate this.
Hope that helps.
Best, SZ [ who would agree "Real Trains" category would be better ]
Edited to add: Here's a photo of a GE test car and the locomotive being monitored; note the cabling at the edge by the walkway stanchions. the locos on the other side are providing a 'load' using their dynamic braking. This was on the GE test track [ on a typical 'dreary Erie' day ], but you could run a similar "all loco" consist on your rr and still be prototypical.