Although not all rail systems in all countries use axle counters for presence detection. In the US freight industry we use track circuits - https://en.wikipedia.org/wiki/Track_circuit

That's not a replacement, that's the older technology.

Axle counting guarantees that there isn't a detached, derailed wagon fouling the line, or a wagon with dirty wheels which aren't completing the track circuit.

Nonetheless, it is what is used on freight lines in the U.S quite successfully, and provides other protections such as broken rail detection.

Detached cars are detected using the FRED/braking system, and secondarily by axle counts given by hot box detectors read over the radio.

This has some really interesting info

https://en.m.wikipedia.org/wiki/Longest_trains

Is there any reason why those 8 locomotives have to form a single train? Can't they just have 8 seperate trains?

A single train only needs one driver, not eight.

Eight trains would need a gap between them ­— typically several km per train — so the capacity of the line is increased by having longer trains.

Air resistance would be higher for eight trains.

Those 8 locomotives can be controlled by a single team of people. Additionally it allows greater freight throughput because you need to maintain minimum distances between trains. It's actually extremely uncommon where I live to see a train with a single locomotive.

Well technically that specific train was a stunt (at the time BHP Billiton's regular run used 336 cars and 6~8 locomotives, now 264 cars and 4 locomotives), other commenters have answered as to why you'd want to run longer trains (and AFAIK modern trains rarely run a single locomotive these days, a pair is just more efficient and convenient even on passenger trains)

See https://en.wikipedia.org/wiki/Distributed_power for more information on running distributed power.

Not to mention, if a loco had a failure the second loco would get the train off the main or to a convenient yard swapping out the failed locomotive.

No? What's wrong with my calculation then?