[corobo]

A train derailing onto another track would be a hell of a derailment

Take this incident for example https://www.bbc.co.uk/news/uk-england-surrey-68466494

Imagine that then resulted in "the following train then slammed into the derailed train, having been unable to stop in time"

[rjmunro]

It happened at the Clapham Junction rail crash in 1988: https://www.bbc.co.uk/news/uk-england-london-46509473

A total of 3 trains were involved.

[corobo]

Oh wow, I was 6 months old when that happened. (36y/o now)

Pretty decent track record if that's the most recent example!

Upping the ante: how much worse would that have been if there were trains following closer than braking distance in both directions.

Special note that the train derailed so severely in this incident after colliding with a train stopped in front of it.

I'm even less of a fan of OP's "braking distance shmaking distance" proposal with this example of why it's a bad idea!