There's going to be good reasons for why a ridiculously long train isn't worth it when it comes to cost-benefit, but these reasons just seem contrived:
> radio reliably
The cars are physically connected, why couldn't you run repeaters down its length?
Presumably any issue you'd have with sightline radio on a >2km long train now you'd also have with a ~200 meter train halfway through a ~50 meter tunnel.
> the brake line systems typically used (air actuated)
And have N independent break systems with central control. It should be more not less safe, a complete failure of one system could be backed-up by another.
> can't see the whole train
So run CCTV on the same system you're using for the radio repeaters?
> radio reliably
The cars are physically connected, why couldn't you run repeaters down its length?
Presumably any issue you'd have with sightline radio on a >2km long train now you'd also have with a ~200 meter train halfway through a ~50 meter tunnel.
> the brake line systems typically used (air actuated)
And have N independent break systems with central control. It should be more not less safe, a complete failure of one system could be backed-up by another.
> can't see the whole train
So run CCTV on the same system you're using for the radio repeaters?