OK, so I did a little math.
- If a station sees about 30 riders per train per day (ons and offs combined), it almost always has checked baggage service. The only exceptions are Lakeland FL and Minot ND.
- If a station sees about 15-30 riders per train, there's a little under a 50/50 chance it has checked baggage service.
- Fewer than 15 riders per train per day - very few stations have checked baggage service, with almost none when ridership is lower than 10/day.
EDIT TO ADD: 30 riders per train is a bit under 22,000 passenger per year, assuming 1 daily train. For tri-weekly-only stations, it's about 9,000 passengers per year.
(For this, I only looked at long distance trains, not any regional/corridor trains which also may have baggage service. Also, I ignored all stations that are also served by regional/corridor trains - for example, I included Cleveland but not Pittsburgh. And I took the average ridership over the last 5 years, hoping to smooth over any serious service disruption that may have significantly affected a year here or there. So for a typical station served by one long distance train per day per direction, the yearly ridership is divided by 730. For a station served only by a three-day-per-week train, the yearly ridership is divided by 312.)