Could the train honk when it's moving in the general direction of the player?

I'm going to look into whether it's possible to implement that - it depends whether current player position and train position are both readable in the API.

I've looked at the API and I cannot find any way of triggering a train event from proximity to the player.

However - it would be possible to trigger a sound event when a train reserves a path through a signal.

So I could create a special signal entity which triggers a honk when the train reserves a path, and this could be placed strategically to warn players of an approaching train.