The real time feed is missing the real time arrivals data - all the data is:
stop_time_update {
arrival {
delay: 0
}
departure {
delay: 0
schedule_relationship: SCHEDULED
If you pay attention to the name, TripUpdate, you’ll kind of get an understanding of why the ETA isn’t there. The TripUpdate only tells you delays (update) for the trip. You then have apply the delay to the trip from the GTFS files that you would have downloaded (stop_time.txt, trips.txt, calendar.txt, etc) yourself.
I’ve never seen ETA present in the trip updates - hope this helps!