The problem is that the train is called different than the base entitys (base: locomotive | Speed Train: speed-train).
There are 2 options:
1. The author of the Bigger Slower Trains mod ZwerOxotnik would add a dependency to my mod and add those entitys to his mod (name = "speed-train" and name = "speed-cargo-wagon"
2. I fork the mod of ZwerOxotnik and add my trains myself which results in a own "[OS] Bigger Slower Trains" mod
I'll send ZwerOxotnik a message - stay tuned