Trunk roads are for any sort of road, be it an expressway, urban arterial, 2-lane country highway, etc., just as long as they meet a certain criteria of importance. For example, this section of 146 links two freeway sections which in turn forms a major heavily-travelled route between Providence, Woonsocket, and Worcester. See here for info on trunk highway tagging in the US: https://wiki.openstreetmap.org/wiki/United_States/2021_Highway_Classification_Guidance#Trunk

Additionally, here is the trunk road section of the Rhode Island highway classification guidelines, which identifies this route as a top-priority non-motorway corridor which should be tagged as trunk: https://wiki.openstreetmap.org/wiki/Rhode_Island/Highway_classification#Trunk_roads