This is a severe issue with the game in its current state. We don’t have an easily referable metric by which to measure travel time. If you introduce a hard limit, you also have to create idiot-proof tools to reference that limit. How are we supposed to create an efficient transport network if we cannot tell time? Right now it’s pretty much a guessing game.
For example, I have a freight train that delivers oil to a refinery with a 12 minute frequency. I have trucks that deliver goods to a city with a 60s frequency. Yet nothing is produced. Clearly this implies that the 20 minute travel time limit is exceeded in this scenario, but there is no way for me to ascertain the actual travel time in the chain from available data.
Often when I encountered such a problem the reason was that the city was already fully supplied with goods by other lines. You can check on the details tab of the city window how many goods items it wants.
Over 5 years, the demand in the city was 72, the city was not getting goods from any other source. I’ve since shut down the line and tore down the infrastructure. Maybe it was a bug, maybe it was something else, but the fact remains – there is no easy way to figure out the overall travel time. Frequency is just too generic and ambiguous.