This is a wonderful place to visit, for kids both young and young at heart. I have been visiting this museum since I was a child, and I think I'm enjoying it the most now as an adult.
I will admit, my favourite part of the visit was the miniature railway. When we first saw the track size we weren't sure us adults would fit, and we were puzzled when a small locomotive came around the bend pulling what looked like a bunch of wooden gym benches. We shouldn't have been concerned, it was a lot of fun! It wizzes through a wooded area, over small bridges and even through a tunnel.
There is also an operational streetcar to take you from site to site, or you could just sit and do the loop and enjoy the ride. Inside the main building are all kinds of trains, from old steam locomotives to schoolhouse cars to sleeper cars. There was even a prototype of the new metro cars we'll be getting eventually. My favourite piece on display though was a open air streetcar used to tour Mont-Royal and Westmount, that looked a little bit like a saute moutons type boat, except with ornate details that include beavers and maple leaf decorations.
Only a short drive from the island of Montreal, Exporail is worth the drive!