The issue with being able to get infinite amounts of energy out of a portal can be solved by thinking with entropy.
If you only look at the entropic system of the portal and the object falling through it, say for instance a magnet inside of a vacuum tube that is perfectly abutted to both ends of the portal, the energy that you are able to extract from the system using a set of coils would come from gravity, and that gravity energy would come from the Earth.
In that case, one of two things would have to happen, either the mass of the Earth would decrease by being converted into energy, or two, the amount of energy needed to maintain the portal would be equal to or greater than the amount of energy that you're capable of extracting from it with an infinitely falling magnet and coil.
I think that the tram would not have to slow down, and that there would be no additional momentum added to the victims as they pass through the portal, and any energy that is lost in those two equations would come from the energy needed to maintain the portals operation.