I was team A, but after thinking all morning I'm team B

Imagine the exit portal is on the back of the trolley, it doesn't even have to be a portal, even just imagine it's an open hole in the front and back of the trolley. As it approaches you jump up, enter the front and exit the back without landing, you then land where you started. See how in this case your momentum doesn't change because both portals are moving at the same speed. If the exit portal isn't moving, you'll gain that momentum.