IIRC - The "Front Runner" was the two axle spine car, the "Four Runner" was simply 4 of them drawbarred together (drawbars reduce slack and lower cost). Big problem is that they wouldn't stay on the track if empty except on the very rear of train, even loaded they had to run on the rear of the train. they might have worked well in some dedicated smaller train service, but Canadian/US Class 1 railroads demand "one size fits all" and really aren't interested in having equipment restricted to a specific service. Notice how much RoadRailer has struggled and never became universal.

I knew a former ATSF/BNSF carman. He said that rebuilding the 10-pack Fuel Foilers had been under consideration, but after a serious look at them it was determined that they were completely worn out and had frame issues (cracked welds), so it was cheaper to scrap them and replace them with new equipment. What was the 10-pack era?

For TOFC equipment history, I think you really need to find what the legal trailer length was, and how the railroads reacted to it. When the 85-89' pig flat was developed, the 40' trailer was common, as was "circus" loading/unloading ramps. I remember both the Night Coast and SDX had plenty of pigs on the rear into San Diego for the ramps into the 1980s, and there were persistent rumors that Santa Fe was going to build a pig yard on Miramar with straddle cranes. The railroads evolved from the concept that any sizeable yard would have a ramp, to only the larger yards, now to just a handful of hubs.

When the trucking lobbies got lawmakers to extend to 45' trailers, TTX and others reset the hitches and could just barely squeeze two 45' on a single pig flat as long as neither had nose reefers. When they pushed to the 48' trailers (and containers), that kind of left the railroads in a bind for equipment. Plenty of new well and spine cars were built to handle 48'. At some point TTX drawbarred two pig flats together and reset the hitches to handle three trailers, with the middle trailer resting on the hitch on one car and its wheels on the following car. These quickly caught your attention rolling a train by as initially something that was about to go very wrong until you realized what it was.

The railroads invested heavily in 48' equipment which in hindsight was a bad idea. I don't know what the economic life of a container or trailer is, but it doesn't approach the 40-50 year railcar life. Truck trailers and domestic containers seemingly only paused at 48' before getting to 53' which quickly obsoleted much of the railroad's previous investment. I know TTX actually built some spine cars to handle 57' trailers or twin 28' per platform, but I've only heard of Canada allowing some limited use of 57' trailers. Import/export traffic (both ships and other countries road/rail) has been far more restrictive to expanding length, so seems to have kept to the 20' and 40' container (where do 45' fit in?)

It's interesting that the rapidly changing intermodal markets have caused the railroads to continually reinvest in new equipment, yet the railroads continue to fight over such traffic that is heavily competitive between rail and road, and has some of the lowest profit margins. I've seen fairly modern spine cars being scrapped, the trucks and brake systems being reclaimed for other uses. The crash in the lumber market has caused similar issues - I know some 1990s centerbeams were just sent to scrap after years of storage. And look at all the 48' wells - shortened to 40', stretched to 53' or scrapped. The only ones I know of in steady use are 48' trash containers on the EVEROO to Roosevelt, WA which is a dedicated service.

As rail traffic continues to recover and even grow, the railroads are going to face some car supply issues. Much of the recent production has been aimed towards frac sand hoppers and oil cars, regardless if the energy market is boom or bust the DOT-117 tank cars need to be built to replace earlier designs now deemed unacceptable for some haz mat traffic. I think all the former UPFE/SPFE reefers are about hitting their 40-50 year life, and have heard of car shortages as potato traffic isn't really suitable for the hi-cube cars they have been building. I'm hearing of similar issues with boxcars, the railroads having disposed of many of the 220k 70 ton boxcars, and there not being enough of the newer TBOX/FBOX type cars to fill orders.

Also hearing some interesting rumors - UP wanting single/loose car traffic back though their yards are incapable of switching anything efficiently, but UP may be back in the shortline spinoff/contract switching market. BNSF wants MRL back. I don't foresee any huge changes in intermodal unless the import market or overall economy is disrupted, but I'm not sure we will recognize today's railroading in 5-10 years.