Trains have a belt and suspenders approach to safety and FRED would go nuts if separated from the locomotive.

https://en.wikipedia.org/wiki/End-of-train_device

Also very few non-railroad people understand pneumatic safety brakes, and suddenly breaking a brake hose locks the breaks on both halves of the train until the tanks drain completely down. Pneumatic brakes operate like a mechanical differentiation analog computer and the strength of the brake application is the rate or slope at which the brake hose pressure is dropping.

An axle counter is like the stupidest defect detector imaginable. Modern defect detectors are like robot Q+A inspectors, every 20 miles or so they detect hot bearings, any load hanging out or down or too high, fancy ones can do on the fly weighing (hmm that tank car full of oil is losing 50 pounds per mile, someone has a leak...) for decades there's been work done with various barcode and RFID schemes to track and log individual cars, pretty interesting stuff. I would guess there exist dumb defect detectors that can not detect overheated bearings at exact integer multiples of 1024 degrees F or something. There's a nice one near a park a few miles from my house and using a scanner tuned to railroad frequencies you could hear a synthesized voice identify itself and say something like "no defects found" I never heard it say anything else.

There is nothing wrong with defense in depth. It was paid for in blood, after all. The biggest problem with defense in depth is it leads to lazyness, well, we don't have to care about 256 axle trains because there's at least three other safety systems, etc etc. Then you end up with every one of them in a corner case and someone gets killed resulting in more regulation.

Depending on the RR, the hot box detector will read out over the radio whether it has found a defect, what axle number it counted it to be on, and the total number of axles counted. It uses a pyrometer to "measure" the temperature of the bearings as they go by, although they aren't particularly accurate and are subject to variances due to environmental conditions. If the detector indicates that it has found a hot bearing, the engineer will stop the train, walk and count back to that axle, hold a special wax crayon on the suspect bearing, and if the wax melts, they have to set out the car. Usually the hot box detector will be associated with a dragger detector as well, and in some places will have other hazard detectors.

As mentioned in other comments, the maximum train length was limited in Europe to around 670m (therefore the possible axle count is rather limited) which will be increased on some networks and tracks. There are several different implementation of axle counters (for example http://www.scheidt-bachmann.de/en/signalling-systems/interlo...), also train splits in Europe will trigger an emergency break due to air pressure drop in the breaking system. https://en.wikipedia.org/wiki/Railway_air_brake

The note is just a sign of the huge safety margin that modern European railway systems implement in regular usage.