The train capacity is around 300 on average, and that is per train. With up to four trains per day that can sometimes mean that the D&S carries over 1,000 passengers per day. They run as many as four busses, sometimes running two busses for one departure time. At most, the busses generally carry back 300 people from Silverton. Keep in mind, many people take the bus up and the train home. That helps balance everything out. Throw in a few adventure packages with outside transportation, and the majority of passengers still take the train round trip.
Yes, the trains home could be lighter than the northbound trip, but I found that was often because of large tour groups that occupied a full coach which were traveling one way and we didn’t have enough passengers departing from Silverton to make up such a drastic difference. That being said, sometimes it was the other way around and the tour groups would travel down instead of up. All I’m saying is that there isn’t a general rule that Silverton-Durango was lighter. It is very situational, and especially during the middle of the summer EVERYTHING is full. Busses, trains... everything.
Lastly, I often had passengers ask me if they could change to the bus and there simply wasn’t room for them as the bus was already full. They also would have to pay the upgrade if they did choose the bus option.