I travelled by bus from Santa Ana to Metapan, a town which is famous for cheese. “Quesos de Metapán”
Bus 235 in Santa Ana – El Salvador:

But I had other reasons for making the trip to this off-the-tourist-trail town.
I travelled to Metapan to scope out the idea that one can take a little-used border crossing into Guatemala.
Last year, I travelled overland from Mexico into Guatemala. I took a bus from Guatemala City to Puerto Barrios, before crossing back into Mexico at El Ceibo. It went through Rio Hondo, a town with a conveniently placed hotel called Hotel Rio and a Litegua bus stop.
Bus #235 runs from Santa Ana to Metapan. Duration: 1 hour 30 minutes.
A convenient place to catch bus #235 to Metapan is on 8a Avenida Sur between 5a and 3a Calle Poniente in Santa Ana.

Bus 235 at the bus terminal in Metapan:

From Metapan, it is possible to travel to Anguiatu, La Frontera.
Bus 211 from Metapan to Anguiatu (La Frontera), a distance of just 12 km:

From Anguiatu, it should be possible to get to Rio Hondo, a distance of 90 km via Zacapa.
If I got to Rio Hondo, the dots would be connected. The trail would be complete.
Google Maps doesn’t show too much in the way of facilities, though. Start early, just in case you have to wait ages for the connection. Maybe I can do this trip soon.