I'm in Chicago and ride the local Metra commuter trains. I'm not familiar with the GO trains so I hope my experience with Metra will translate somewhat decently for you.

In the mornings, coming into the city, with the mad crush of commuters, it is, of course, best to have the smallest possible fold. Generally speaking, a folding bike, even a Tikit will not be turned away. That said, the larger the fold, the better it is for you and your fellow riders if you kindly bag the bike.

I have the Tikit, and I, fortunately, have not encountered any issues with it unbagged on any train. That said, my ride times aren't during the worst parts of rush hour when space is at a premium.

The realities of rush hour in the big city basically mean that you will likely have to bag your bike if the train is packed like sardines. If it's that packed, it won't matter if you have a Tikit or a Brompton. Sure, the Brompton takes up less space, but you're going to have to find some spot somewhere to put it down. If your local commuter trains have storage spaces (or odd corners where you can stow a bike) somewhere... great. If not, your bike is going to take up the space of another human... even if it is a Brompton.

Your best bet is to try the commuter train during the time you plan on using it to see how packed it gets. If it looks like there is a wiggle room, get the bike that best suits you with regard to ride (some folks like the Brompton, some folks like the Tikit... it's pretty subjective so you'll need to test ride both to be sure). If it looks like you'll be packed in, but there is space to stow a Brompton, get the Brompton.

--sam