Why can I purchase tickets for one route between two cities but not another route between the same two cities?

Why can I purchase tickets for one route between two cities but not another route between the same two cities?

As we offer online ticketing, we are expanding the ability to purchase tickets across the entire Coach USA network of buses and routes, but that takes time. We will be adding more routes each day. If you would like updates, sign up for an account today, and you will receive updates as we add functionality and expand ticket sales to new routes. In the meantime, we show the stops served on each route, so if you click on the information icon, so you can check the stops on each of the routes, and see if your stop is covered by the online ticketing system.