Could you add a new constant combinator that provide the train's current speed for a passed-by train.
Or (i think it will be better) - the speed of the train in a current rail section (between signals):

...ooo>>.... – Train
= = = = = = =
^.... !! ......... ^

^ – Rail signals (chain or regular)
!! – a new constant combinator

Scenario: check current speed of the train, and change next signal red/green for one of two ways.
Example – Y-way rails:

.oo>>........//=^====== – it is a detour for fast trains
============== – way for slow trains only (controlled via circuit)
...........!!..^.......^

^ – rail signals (chain or regular)
!! – a new constant combinator
== – rails

Idea: if someone crosses a railroad track far ahead and the train is very fast (speed higher than some limit) - push the train onto a parallel track (but some distance from the straight track) - this will allow keep train speed, and be in safe to cross the railway (unstoppable fast trains). Also, it will be better jump distance control for train ramps in mod: https://mods.factorio.com/mod/RenaiTransportation

I'm not sure that changing signals (to red) will immediately reroute a train. Maybe it'd help to put a chain-signal in between, as the game engine seems to actually use those when rerouting in realtime. I might look into it, I don't promise anything :)

I don't have enough control over the game-engine for this, unfortunately. So I'm closing the ticket.