This reminds me of an old story (probably apocryphal):
The main thrusters on the Space Shuttle were built and shipped to the launch site by rail. That was actually part of the design specification. Well, railroad gauges were originally based on existing roads. Many roads were simply built over the wheel ruts from old horse drawn wagons. The horses were usually hitched side by side. So when you get down to it, the size of the most advanced vehicle on earth had at least one of its design requirements based on a horse's butt.
How does this effect you? You need to build 2 marshaling yards on each side of the portal. First things through the portal should be some defensive personnel and some heavy duty weaponry. Start smallish, establish a perimeter, and do your best to camouflage what you are doing. Then bring through some small bulldozers, small earthmoving equipment, and so on to build up the yard. Get some rail laid through the portal, and make sure to build the iris barrier (ala Stargate SG1) so that no matter what happens you can close it off if things go wrong.
Once you have the area on the other side established with rail and equipment for you to unload with, you can really go to town. I would guess that two or three rail cars at a time could move an awful lot in a short time. Cargo containers can be used for double duty as well. One intermodal cargo container can carry a bunch of stuff and then double as a barracks or even a part of a substantial wall when emptied.
Now, there are a great many aircraft that should be able to get through afterwords. The Fuselage of a 737 can ravel by rail, and the wing and empanage came be sent on another. I have seen repair crews on some smaller aircraft be able to mate the wing to the fuselage in about a day (a lot of wings are just one piece). Jet engines are surprisingly small. Even very large ones are not any larger than a Volkswagen bug. They look bigger but that's just the cowling. That's just an idea of how big you can go. For something like this, though, I would suggest smaller aircraft, like maybe an AT-6 light attack craft as a place to start. They kinda look old fashioned but they are extremely capable aircraft to get started with and they are fairly small. They can also take off and land on a fairly short, primitive airfield.
While you bring through the basics start building things to be as defensible as possible. Concrete barriers can come through and be quickly stacked. Defense is absolutely critical, because if the the enemy closes the portal, you are absolutely done.