Yes there are detectors at various locations usually of different types or capabilities on most routes.
These can detect hot wheels/axles, impacts(flat spots or broken wheels) axle counts etc.
There is no detector on the actual car to check for defects. This is done by periodic inspection by crews and mechanics, by feel/tempstick, visual inspection etc. and by roll by inspections when one train passes another, or location.
At this time most freight rail equipment only has air line connection between cars for the brakes. They have radio telemetry and communication with remote engines in the train or end of train devices, to control the engines or brakes etc. But in that the entire train is controlled by air braking. There is electronic braking being developed but it is many years away from being actually in use.
If you see a standard train with a couple engines on the front and just a bunch of cars, the engineer is controlling the engines which are direct linked together with cables and air lines, then the rest of the train has a air line run the length of the train for brakes. Finally, there is a end of train device that is radio controlled to give data to the front end on what the rear end is doing, air pressure, movement, etc and to allow the front end to "dump the air" from the rear end. This puts the train into emergency braking, from the rear, which can be done from the front as well or both. But can cause catastrophic results depending on the circumstances. But in an absolute emergency you dump it all from everywhere to do whatever you can to stop that train, but usually it is better to bring it to a controlled stop and not risk derailment and/or tearing everything apart and usually doesnt stop any faster.
