I had exactly same issue with same model.
The battery box
HUB NO. 4 was blinking green (colour of connection) and orange.
I changed to brand new batteries.
Still the hub was blinking green (colour of connection) and orange.
As they mention above it's due to the batteries is out.
What I like to add and to highlight is that there is x6 batteries to change.
LEGO - Powered Up troubleshooting - Changing batteries
Changing the batteries in your LEGO® Power Functions Train
- Remove the LEGO Power Functions Battery Box from the train.
- Remove the two screws on the bottom of the battery box.
- Replace all six batteries with fresh, high quality AAA alkaline batteries.
We don't recommend mixing brands or old and new batteries.
LEGO - Building instructions for 60198 - Book 3 pdf
See page 9 instruction 2.
In my case I been to lazy and didn't check any instruction.
I only open the box, change the 3 batteries I saw and mount it back.
After changing all of the x6 batteries issue solved.