Funny story: I tried improved breakdowns on my slow game today. It's a game I started too early; I have only one engine and it only does 43mph. It can take trains 6 months or more to get from source to destination. To handle all the cargo, I have a lot of trains, there's a lot of traffic. With improved breakdowns on, even after the breakdown clears, a train may be stuck doing less than half its maximum speed until it's next service. What happens? Well, I've had queues 300 tiles long, 150 tiles back to a drop-off station and 150 tiles behind it. The industries are FIRS and I make heavy use of engineering and farm supplies to boost production. When a supply train is stuck behind another or is itself stuck doing less than half its maximum speed, what happens? Production of the affected industry drops to 1/3, meaning all the trains needed to support its full production queue up waiting to enter the station, which itself is likely to inhibit the supply train reaching the station...
Fortunately, the latter situation hasn't actually triggered devastating queuing, but the potential is there for this one feature in this particular game to be worse than any other disaster the game offers!
I can patch up after a destroyed UFO far quicker than it took that 300 tile queue to clear! It's a really big event when this happens!
Edit: Of course,
when a train is running so slowly, it has more time to break down again, too!
I tend to give vehicles with medium/long order lists more than one "service at depot" order.
This way if they do break down or otherwise need servicing they don't usually have to go too far before the next service order.
If you have queues that long resulting from a single breakdown you probably need more slack/capacity in your layout.
When I use trains with full load orders, I often place some queuing sidings before the loading stations so that if trains come in faster than cargo is available to be loaded, they can queue up there without blocking unrelated traffic, or worse blocking traffic trying to leave the loading station.