I have come close a few times, but never complety had a train all to myself.
TOP GUN(PKI) Went on this ride after SOB,It was a complete walk on and after entering the station, It was empty and Hopped in the back, I thought I was gonna get a Ride all by myself, when about 4 more people came in and went in the Front, so overall, It was just Me and 4 other people on a ride. Kinda Fun, Done it later in the day too, when there was about 5-6 other people in the train.
ADVENTURE EXPRESS(PKI) almost again had a ride to myself, I sat in the back and noticed only 4 other people on the Ride, they filled The Front Car.
OBLIVION(AT) This was in the first year when Oblivion opened, We started Queueing Up, but then they made people queue down and around Engergizer to control the crowds, me and my friend were mad as we were First in Line with a Dad and his 2 Daughters, so we all conplained to a manager, we were gald we did. He let us stay where we were, so we could have the first ride of the day, anyway about 5 mins till 10AM, we were lead up to the entrance(Just me, My Friend and the other people with us) anyway, when we get into the station, they let us board a train immediatly,but where was everyone else, Surely they wouldnt Let just 5 People(Me, My Friend, The Dad and 2 Daughters) Ride Alone, after a short while, they closed the gates and then we realised we would be Rideing Alone(Just the 5 of us) A Pretty Awesome Ride with only us 5 People in The Front Row. Also as it was the First year it opened, They had the Steam over the Tunnel covering it complety, It made it a lot better. Now though, they dont do it for some reason.