I think I related elsewhere about my most recent experience of this. I was waiting for the bus to Bodmin Parkway at Bodmin General station. The bus I was waiting for was on the screen and the 'due' time commenced to count down in minutes. When it got to zero the bus hadn't arrived, and it then disappeared off the screen. Ten minutes later and the bus still hadn't arrived, so I started to walk down the hill towards the town centre, hoping to find a taxi. Then, guess what.......
Did it display x mins gradually reducing or show a fixed time before disappearing of the screen?
RTPI is reliant on a number of factors. Accurate schedule data, gps tracking working, rural mobile signal to transmit the GPS positions and so on.