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.