As supplied, the railbus exhaust is just a bit of tube. To my mind this didn't seem right. Surely the rain would get down it?
What we needed was one of those funky tops that bob up and down in the fumeflow, shutting completely when the engine is off. Like those things you see on trucks.
A little bit of noodling around with some brass sorted it. I wrapped the top of the tube in a thin bit. A length of tube filed down would have been quicker but some bodgery with files and solder looked OK.
The cap was cut with a leather hole punch - it takes a bit of waggling the metal around to cut it but the result is circular and slightly domed. A bit of wire holds it up and slightly open.
At the bottom, a bit of tube and another punched circle beefs up the base of the pipe.
Probably an hours work while watching TV, and a very pleasant way to spent some time.