I’m a scale guy, but I do have my son’s Railking double stack car here and I think it’s a good option for semi-scale intermodal. It’s nicely proportioned and compressed with same detailing as the original issue Premier cars. Here’s a photo comparison:
Intermodal is a lot of fun and there are a lot of variations in intermodal operations. Sure, there’s solid stack trains and priority TOFCs, but mixes of stacks, TOFC, and even COFC happen. Even autoracks sometimes are mixed in. And intermodal cars come in different sizes and styles. I think the Railing car would fit well with your Lionel cars as would the other options mentioned, including the Lionel spine cars.
I have a few of the old ROW cars, which I use on my scale trains. They have wells large enough for scale 40 foot containers (or two 20s) and bigger containers can ride on top. Very prototypical:
As “Curve” mentions above the old ROW cars are now available from Menards. They size out between the Railing and Premier Gunderson style cars:
But remember there’s variation in prototype sizes. I’ve not seen the Menards car in person, but to my eye the containers that come with it look to be slightly less than scale (unlike the original ROW containers). But that’s likely better if you’re mixing them with other semi-scale equipment.
Like the Lionel cars mentioned above, the Railking car is probably about 1/64 scale in size. I tried a Lionel starter set trailer in the well (also prototypical) and it looks just about right compared to a 1/48 trailer in the Premier car:
By the way the upside of going semi-scale is that 1/64 is a preferred scale for die cast truck models and so there’s a wealth of trucks and trailers available to complement your trains.
Good luck and have fun.
RM