Hi, just checking, is this happening the first time a train goes across the route, or after the first time? Trains remember the speed limit they had between paths, so if one path sets a low speed limit and they never go past a faster speed limit again (or don't see it on their route somehow) then they'll continue to go at the slow speed. If you want them to speed up again, you need to put another speed limit sign that the train will go past, with a higher speed limit.
I hope this helps!