Time to share more secrets 
If civ A and Civ B have a trade route between city A and B if you place any kind of unit(ie caravan, pikemen etc) Anywhere along the optimal path between those 2 cities the trade routes will be cut in half(road railroad bonus's are droped). This means with a size 50 route it will turn into size 25 or less.
How ever if there is another city on the optimal path, it gets a little more complicated. If a rome route is going to babalyon and Veii happens to be in between then only way to interupt that trade route is to put a unit between veii and rome.

If civ A and Civ B have a trade route between city A and B if you place any kind of unit(ie caravan, pikemen etc) Anywhere along the optimal path between those 2 cities the trade routes will be cut in half(road railroad bonus's are droped). This means with a size 50 route it will turn into size 25 or less.
How ever if there is another city on the optimal path, it gets a little more complicated. If a rome route is going to babalyon and Veii happens to be in between then only way to interupt that trade route is to put a unit between veii and rome.
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