The fastest factory station wagons?
It's probably some rare late 60s beast from Mopar with either the 440 or a 426 in it.... but where will you find *that* documented??? There's probably a big Mercedes or BMW wagon that's got beans and has done well over 3 digits, but again, where have you ever seen *that* tested.
Then there's the good-old-USA wagons, with one model in particular. That's right, the ultimate sleeper wagon, the 1994-1996 Caprice wagon with the B-platform-car version of the second generation LT1 (260 hp @ 5000 rpm and 330 ft-lbs at 3200 rpm, both net figures). I know what you're thinking, sure that powerplant would make quite a fast car given that its sedan brother ran 140-142 mph, but not with the non-speed-rated tires and not with a 108 mph speed limiter. I'd agree with this, but then I'd remind you that Chevrolet made a "special service vehicle" version of this car in 1994 and 1995, calling it the 1A2... and the Michigan State Police tested one in late 1993 and 1994 (the 1994 and 1995 model year tests) during their annual tests of police and special service vehicles held each year in the Fall.
The 1994 Caprice 1A2 wagon topped out at 132 mph, while the 1995 version (tested in hotter weather) ran 129 mph. Those tests were conducted at the Chysler Chelsea (MI) proving grounds, if memory serves, on a 4.7 mile long oval track, and allowing 4 laps where the max indicated speeds were recorded from the calibrated speedos that each car had. That's also tested with a full tank of gas and two cops (driver and passenger) aboard.
The 1A2 wagons were also tested in 1991-1993, but these had either smaller or less powerful engines, so they didn't run as fast as they did for the 1994-1995 model years. But they sure looked cool running on the banked track, per the above picture sporting Chevrolet Police Vehicle logo and black-and-white paint motif.
The civilian model 1994-1996 Caprice wagon and 1994-1996 Buick Roadmaster wagon both got the same 260 hp 5.7 liter LT1 motor (the only one you could get in a wagon) that the 1994-1995 1A2 wagon got, so the only thing(s) preventing these civilian wagons from running close to 130 mph (aside from a long straight dry safe road, and ahhhh no law enforcement) is speed-rated tires (H-rating or better) and a powertrain control module (PCM) reprogrammed to defeat the speed limiter. I suspect the Roadmaster might be a tick slower than the Caprice due to the Vista window in the roof and the extra aero drag that goes with it, but that's the price you pay for Roadmaster (RoadMonster?) style. One "modified" Caprice wagon that I know of, containing a genuine retrofitted Corvette LT4 engine, has seen in excess of 145 mph.
"We ordered the car in December of 1990 and took delivery of it in March of 1991. We ordered it in all black with a gray interior. The car is fully loaded and even has the level ride in the rear. When we got the car we pulled off the white walls and turned them inside out with the black out. We then took out the grill and painted the egg crate part of it black. To say that it no longer looked like a Bingo car is an understatement! In 1994 when the Impala SS's came out we really did a number to it. We got a set of the OEM Impala wheels and tires, then we put in the Impala grill the logo on the rear quarters. The tail gate then got the Caprice removed from it and in its place the running deer was installed. In the interior Caprice over the radio was removed and guess what - the Impala Logo will fit right in place with no mods to the dash. The side chrome and trim was removed from the car and the Impala molding was placed on the sides of the wagon. The front bumper cover chrome was painted and the rear bumper was removed. In its place is a MOON rear cover that looks like a roll pan. It was a lot of work but we believe that it was worth it. We believe that the car looks like the SS WAGON that Chevrolet built as a prototype and should have built for retail."