When there was no Ma, though, the crowd was quite low. A short show might be indicative of an anticipated crowd, rather than an actual crowd. Would that be accurate?
Nope, it's based on either how many people are actually on the train, or the schedules they are running. They won't go to the 20 minute schedule until they are leaving with a full train and have 50-75 people left behind that couldn't get on. If they aren't leaving people behind, they will stay on the longer cycle of a train every 30 minutes and the long show.
If the crowd was quite low, then more than likely there was not a good candidate to be Ma on the train (there are certain things that need to be place for it). If there's no candidate, they will skip ma.
If they are running on a 20 minute schedule there will be no ma. This is usually because it's busy and they are trying to get in an extra train every hour (3 instead of 2). However if things have happened during the day to put their time table off, they may run a 20 (no ma, short show) to "catch up" and get back on schedule.
This may have changed now, but on really hot days they might sometimes run 20's/ short shows just because it is so hot that people on the train are miserable and not enjoying the show, so they cut it short to "put them out of their misery." There's no reason to do a long extended show with Ma if the passengers are not enjoying it and not into the show at all.
The last train of the day is ALWAYS a short, no ma show.