RedRocket, would you happen to know what route the double decker is on as it passes through Bramalea Station and if it is being exclusively used on that route (as opposed to being mixed in with the regular coaches)?
They only operate on two routes, but they are mixed in with regular highway coaches. You have a much higher chance of catching one of the 45 Streetsville/Square One/York U route in the peak hours, (as opposed to the 46 Oakville/Square One/Bramalea/York U route) but I haven't seen enough consistency to be able to predict which runs will have which buses.
You can call the GO hotline to ask for when the next Double Decker will arrive at your station. Apparently they get these types of calls fairly often and some riders will wait for a Double to arrive.
I really like the buses, I predict it will soon become an iconic part of the GO Transit system.