Add a Way to Read Train Trash Slots

[danbopes]

I want to setup a condition on my refueling station, but noticed there isn't anyway to read what is currently in a train's trash slots.

My goal is to use a condition:
Trash Slot Ash = 0
Coal = 150

Currently, I have to remove ash via an inserter, and then use an inactivity condition, which is a little wonky (If the chest fills up, it could cause the train to leave unexpectedly). There's also no way to send trains to a stop if their trash slots fill up.

I request some way to read fuel trash slots.