Well you are sorta true. They run three cars during haunt but on they also do three cars during the beginning of the season when the park doesn't have a lot of visitors. This year, Colerain had an off day on a Tuesday at the early in the 2013 season so we went up to KI and they were only running one train with three cars. We sat in the 3rd car.
The reason for this is because the PA system is in the front car along with the wheel chair section. In years past, up until 2012, the wheel chair car was in the back of the train and the PA system where the conductor speaks was in the front. Now in years past for the fall season, they always ran 3 cars: the front PA car, a middle car, and the wheelchair car, where the other conductor stood.
Now they can just run as many cars as they want because the rear conductor can just stand in the back of the car.
As for the cab, like what Adventuro said, you can only get on when the train is parked.