Heh, I was going to Central station in Glasgow on the day the 'choo-choo I'm a train' bus was there and have a picture of it on my phone. The driver tried to charge me money for taking a picture of his handiwork as he was so proud of it.
There's often a replacement bus service running between where I live and Glasgow on a Sunday for some or all of the route and I quite regularly pass a bus with that message on my way home from work. I think it is the message they are told to display rather than a one off joke. Still great though.