Colectivo from Zihua to Troncones

Your best bet is to get yourself downtown to Benito Juárez, the Main Street where the mercado is. Avenida Las Palmas is between the mercado and the ScotiaBank. On Las Palmas, just east of Juárez is the depot where the micros start their trip to La Union. Get onto the micro there.

Any time I have done that, I have gotten a seat. Any time I have got onto the micro more than three blocks from the starting location, I have stood most or all of the way to the Troncones drop off location.