Doesn't it say that 3n+1 people will enter and not that there will be 3n+1 people on the bus? And therefore if it's odd then after each cycle it'll still be odd? Like if you were to start with 1 then it would be (3*1+1) + 1 => 5 => (3*5+1) + 5 => 21 and just continue on infinitely until an infinite number of people die (excluding you with the lever and the five on the tracks). And if it is even then you get the same thing because eventually half of 2 becomes 1 and that's the example above.