First of all, The description isn't exactly true. Right description should be:
Complete a level with 10 simultaneously running trains.
You can stop them, it won't prevent the achievement to pop!
It's a tricky achievement, I did it on 1-1 map, I think it's the best map.
I just built a lot of tracks to stack my trains without sending any of them to their destination.
You'll also have to call some additional trains to reach 10 trains on this small and quick map.
When there are 10 trains, time will surely come to an end, just deliver each train to their station and achievement will pop! (You can also crash them, to achieve quicker, but avoid lack of money, I'll run out of money fastly trying this achievement!)