The RR line on which the collision near Mendon, MO occurred is the furthest thing from "infrequently-used tracks"; these tracks are part of the "BNSF Southern Transcon[tinental]" line[0] between Los Angeles, CA and Chicago, IL, which carries (per previously linked Trains Forum thread: "Posted by tree68 on Thursday, June 30, 2022 7:59 PM") "The FRA/DOT crossing data shows over 50 trains a day..." (wikipedia[0] provides a higher number). And the vast majority of the BNSF Southern Transcon in MO (and elsewhere) is double-tracked, meaning trains can (and usually do) run near full track speed (90 mph in my understanding) without the frequent need to stop for opposing-direction traffic as would be typical on single-track lines.
Also, I don't know if you were applying the term to the road of the crossing involved in this crash, but this road is not one I would characterize as "well used"; more like "exceedingly rarely used" (thus the lack of active crossing protection).
Finally according to the previously referenced Trains Forum thread ("Posted by blue streak 1 on Wednesday, June 29, 2022 11:32 AM"), the railroad existed 60-70 years prior to the road in question. I believe it is this circumstance which places the onus on MODOT to defend the users of their later-arriving road from hazards associated with rail traffic on the preexisting BNSF line (with BNSF being an involved party).
I understand citing posts another public forum does not necessarily meet the gold standard of citations, but all of these facts align with my preexisting understanding of the situation.
[0] https://en.wikipedia.org/wiki/Southern_Transcon calls out Mendon, MO as a station on the Marceline subdivision