I would say that "this bus is the one that stops everywhere" (and takes forever), compared with the Express. The "cata-coió" reminded me of the northeastern expression "pinga-pinga" also referring to such buses that frequently stop to pick or drop off passengers.
Compared to mail (snail mail) perhaps we could
by analogy say
that they are kinda "snail bus", because of it´s snail pace.
But then, it´s a bit of poetic licence here.