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I-70 west of Denver, from the Continental Divide (Eisenhower Tunnel) west to Silverthorne, is 7% all the way. With two runaway-truck ramps... that get used frequently.
Drove it a whole lot, in my '69 VW bus when I lived in Dillon. That teaches you patience.
Technically there may be steeper Interstates, but your original guess is a good one. I-84 in Northeast Oregon (Emigrant Hill, just east of Pendleton) is one of the most magnificent stretches of 6% grade in the US. Multiple lane, multiple emergency truck escapes (westbound), is a great example of steep freeway construction in the U.S. Totally rebuilt in the 90's, this is Interstate freeway at its best. If you have a pulse, this drive will raise it, especially westbound coming down the hill.
I agree! I drove this just before Thanksgiving this past year in a big Penske moving truck after dark, and it was an interesting experience. The sharp turns, the steep grade (was headed north towards Pendleton, so it was a steep descent) with the city of Pendleton lit up down there in the lowlands. Hands were sore from holding the wheel tightly, but it was a great road!
There is a highway the name of it is fm 170 in Texas on the Rio Grande river it has some grades on it rated at 18%. The Dom rock from the Kevin Costner, Judd nelson movie Fandango is on that road. The highway is west of Lajitas Tx east of Presidio tx.
In addition to the grades already mentioned, I'd think that I-15 from Mountain Pass CA to State Line NV warrants some mention (and probably from Halloran Summit to Baker CA).
But the Tejon Pass/Grapevine on I-5, and I-80 east of Salt Lake City are the first big grades that spring to mind for the interstates (US 101 on the Conejo Grade is also very steep but not really relevant to the "steepest interstate" question).
Although certainly not a plaque carrying expert, I have worked in the industry. My UNDERSTANDING is 7% grade is the maximum allowed in the US Interstate System, since it was originally conceived with military implications. We them here in Colorado too. Does anybody know anything to the contrary? Other types of roads and highways may not be so strictly regulated.
The Cajon Pass/Summit on 1-15 outside of Victorville, CA. It connects Los Angeles to the midwest, south, and Las Vegas. If you are coming to California from Kansas or midwest (no matter which route you go) you eventually end up on here. It is a very steep downgrade that lasts for 12 miles to the 15/215 interchange. IT SUCKS. I was not prepared for it as I had researched all the other downgrades, but overlooked this one. If you plan on driving form to Las Vegas from California, you cannot avoid this. It freaked me out. Cars were passing me left and right going at least 75-80 mph. I refuse to drive it again.Either I am a passenger in the car or I fly when I go to Vegas. It is a steep downgrade and while I am sure there are others that are even steeper, this downgrade last for 12 miles you are descending for 10-15 minutes. Uggh!!
What about the steepest grade for a railroad line? I think that might be somewhere in North Carolina.
Nope, Cog Railway, goes striaght up Mount. Washington in New Hampshire.
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Cajon Pass/Summit
It connects Los Angeles to the midwest, south, and Las Vegas. If you are coming to California from Kansas or midwest (no matter which route you go) you eventually end up on here. It is a very steep downgrade that lasts for 12 miles to the 15/215 interchange. IT SUCKS. I was not prepared for it as I had researched all the other downgrades, but overlooked this one. If you plan on driving form to Las Vegas from California, you cannot avoid this. It freaked me out. Cars were passing me left and right going at least 75-80 mph. I refuse to drive it again.Either I am a passenger in the car or I fly when I go to Vegas. It is a steep downgrade and while I am sure there are others that are even steeper, this downgrade last for 12 miles you are descending for 10-15 minutes. Uggh!! 
