IMPACK is a brand name, spine car is a generic term. Technically, all of the IMPACK cars are spine cars, but that is not true in reverse. 

Some history of the spine car... 

In 1978, ATSF designed and built the 10-unit Fuel Foiler, which was designed for 40' & 45' trailers and run strictly on ATSF, as they were not designed for interchange service. In 1981, Itel (FMC Gunderson) bought the design from Santa Fe and dubbed them "Impack" (Inter Modal PACKage). Itel sold the cars in various configurations, 3-, 4-, 5-, 8- and 10-unit configurations. ACF and Thrall also built similar designs. Aside from Trailer Train (reporting marks were UTTX originally, then TTLX on later designs), BN, SP, and SSW bought these cars. Additionally, they were now designed to handle 40', 45' and 48' trailers. The later TTLX design cars were also able to handle 28' pups. [Athearn and ConCor(?) have produced a version of the early car, which handles 40' and 45' trailers, UTTX.] 

In 1987, the (NTTX) container-only, articulated skeleton car, came along as a five unit set, which is able to handle 20', 40', 45' and 48' containers. The builders of this design included Bethlehem, Trinity and Hyundai. [Alan Curtis produces a brass kit, which is limited in production.] 

In 1988, the All-Purpose spine car was released with the design to handle 48' trailers/containers, carrying TTAX reporting marks. The end two platforms can carry two 20' containers. Bethlehem, Gunderson, Trinity and Thrall were the builders of choice. [Walthers released this car as a kit and then RTR, but is currently out of production.] 

In 1993, the TTAX all-purpose spine was lengthened to 53' and can carry 28' to 53' trailers. On the two end platforms, there is the capability to carry two 20' containers. Prototype builders are the same. [Alan Curtis has a limited run brass kit available.] 

(Note that AC is getting out of American prototype models and it is my understanding that someone else is taking over at least the N scale line in the near future. AC might be producing one last HO scale car. Production was scheduled to include the TR / TTRX 3-unit 6x28' / 3x57' spine car. Not sure if this is still the case.) 

There have been a fair number of the 48' TTAX cars that have been stretched into 53' TTAX AP spine cars. Additionally, TTX has recently purchased 3-unit 53' AP spine cars, rolling with the TTAX reporting marks. You can find TTLX cars still around, I've seen four sets here in Atlanta within the past year. 

I knew they were TOFC just didn't bring up my process for that yet and think cardstock may be the best way to go. 

If you can find a copy of December 1992 issue of Model Railroading there is an article "Spine Cars - A prototype Overview" by David A. Bontrager.

Additional references:

Model Railroading Dec 97 article called:
"Spine cars part 2: single purpose minority owners" - Author Doug Geiger, MMR.

Book, "Santa Fe Heriatage" by Steven & Cynthia priest, chapter on these cars, plus lots of (mostly publicity) photo's.

Text from the magazine article:

Santa Fe (ATSF) Owner

These were the first successful articulated spine cars that rode on conventional trucks although in captive ATSF service, these cars proved the way for many of the later spine car designs. The first car was an experimental car built by ATSF shop forces in Topeka KS. It could haul six 40' or 45' trailers, five faced the B-end of the car and one faced the A end. The trailers had to be lifted on & off each platform unlike the circus-style loading required of many vans in intermodal service at that time. The hitches were Pullman Standard non retractible. In summer & fall of 1977 this "six pack" car made a splash with the railroad industry if the amount of magazine coverage is any judge. Painted white, the car had a very large air-supply reservoir plus a triple valve and brake reservoir on both the A and B units, and ran quite low to the ground, it carried the number 298999.

The first production run of 10 packs was numbered 298988-298998, built 4/78, coded FT-93, the second was 298946-298987, built 3/80 - 7/80, coded FT-101. The prototype 298999 was rebuilt into a 10-pack in 1981, but kept many of the odd features (upright brake wheel, large platforms) although the brakes were apparently simplified to match the other cars. The cars only had a brake wheel on the B platform. Paint was white and the hitches were highly visible black.

Some modifications happened over time, with the arrival of 102" wide trailers, extensions were added to the platforms, (little triangular bits jutting out) from the pics neither build had them from new, and can only confirm 298965 without them in '84 and 298981 with them in '89.
298965 was rebuilt to haul standard (2x20' or 40' or 45') or A-Stack containers on all the intermediate units (but trailers on the ends) the A stack containers could even be double stacked on this car. This was recoded an FT-103 flat.
298998 was rebuilt to a COFC only car, set up for 2x20'/1x40'/1x45' on each car. It stayed coded as an FT-93.
Both of those cars had a second brake wheel added at the A end of the car when modified.

When repainted most cars lost the "railroad roman" font & got sans-serif Helvetica to match the ATSF practice of the day.
298981 got painted in mineral brown with circle crosses in white. The magazine article also says some cars got the Q logo, but there's no photo's of that.

The article says that as of Feb 92 all TOFC cars were in service, by Feb 96 only 21 remained, and by fall '97 they were all out of use.

There's no mention of them being rejigged in later life, however in '82 ATSF sold the design to Itel, who slightly modified it & started selling it to other railroads in other formats, including:
5 & 10 unit sets to BN
4 & 8 unit sets to SSW

Jim, what you just quoted is the link I provided.