TL;DR
This is like a belt scanner; connect a wire to a rail segment and it becomes a scanner which can read the contens of the vehicle above it, be it rail car or locomotive.
What ?
The creation should be as easy as with the belts. Connect a wire to a segment of rail and the segment gets a scan-option + a visual change to it. The reverse should be the same as with the belts. Now cars or locomotives can be scanned as long as they are on that segment.
Why ?
Its already pretty good that stations can be connected to a circuit network and trains can be serviced with such systems. Yet the read-out only allows to see the contens of the entire train and not single freight cars, which I want to change. The reason is that systems can have more complexity if needed. trains can have different lenghts, number of locos and cars, direction of engine, which could all be seen individually through a scanner under each loco or car (if the player placed them with the right distance inbetween). freight cars can have the same item-contens but the indiviual loading-status can be seen. circuit connections can be made individually for more specialised tasks which arent possible right now or still the same with some simple wire connections. The current station itself could become less complex with options, since those move to the scanners.If someone has this as a mod I would glady try it out or I would propose this as a mod for testing.